Phenolic resin method

Abstract

The invention relates to a method, a device, a binder system, and a material system for producing components using layering technology, wherein the temperature in the building space and/or in the applied material is set to at least 70° C. and maintained for at least 2 hours. Areas on which binder has been selectively applied, solidify and form the component.

Description

CLAIM OF PRIORITY

This application is a national phase filing under 35 USC § 371 from PCT Application serial number PCT/DE2016/000209 filed on May 18, 2016, and claims priority therefrom. This application further claims priority from German Patent Application DE 10 2015 006 363.4 filed on May 20, 2015. PCT/DE2016/000209 and DE 10 2015 006 363.4 are each incorporated herein by reference in its entirety.

The invention relates to a method and a device for producing three-dimensional components. These moulded parts are suitable for use in casting applications, in particular as moulds and cores.

European Patent EP 0 431 924 B1 describes a process for producing three-dimensional objects based on computer data. In the process, a thin layer of particulate material is deposited on a platform and has a binder material selectively printed thereon by means of a print head. The particulate region with the binder printed thereon bonds and solidifies under the influence of the binder and, optionally, an additional hardener. Next, the platform is lowered by one layer thickness into a construction cylinder and provided with a new layer of particulate material, the latter also being printed on as described above. These steps are repeated until a certain desired height of the object is achieved. Thus, the printed and solidified regions form a three-dimensional object (component).

Upon completion, the object made of solidified particulate material is embedded in loose particulate material, from which it is subsequently freed. For this purpose a suction device may be used, for example. This leaves the desired objects which then have to be freed from any residual powder, e.g. by brushing it off.

Problems occur in known methods with respect to the binders used, which often attack the device itself and, in particular, the print head and are in some cases problematic from a health perspective, too.

Also, the further process conditions may be problematic and suboptimal for the production of advantageous components. In particular, the temperature distribution and suitable binders do not always allow positive process results and often adversely affect economic viability.

Therefore, it was an object of the present invention to provide a method by which advantageous component can be produced and which is advantageous in terms of economic viability, or which at least avoids or at least reduces the disadvantages of the prior art.

BRIEF DESCRIPTION

What is described is a method for the layered construction of components, wherein a particulate material is applied onto a construction area in a construction space layer by layer with a layer thickness, a binder is selectively applied, the temperature in the construction space or/and in the particulate material applied is adjusted to a desired temperature, and the steps of material application and binder application are repeated until a desired component is obtained, wherein the temperature in the construction space or/and in the material applied is adjusted to at least 70° C. and maintained for at least 2 hours, with the areas onto which the binder was selectively applied solidifying and forming the component.

In this context, it is advantageous that an unproblematic binder system can be used, which is water- or alcohol-based as a solvent and which is adjusted to and kept at a temperature of at least 70° C. in the applied particulate material over a long period of time. This has the advantage that, using this method, a very large area in the applied particulate material thus maintains a homogeneous desired temperature, which in turn allows uniform solidification and advantageous component properties to be obtained. This has the advantage not only that homogeneous material properties are achieved in the component, but also that, advantageously, a large area of the construction space can be used to produce components and the unused edge region of the construction space provided with particulate material remains relatively small. This increases the efficiency of the device and thereby reduces the cost per component or high-volume component, respectively. The binder system used in the method is preferably only slightly reactive at room temperature and the machine parts and, in particular, the print head are easy to clean and allow maintenance without any notable problems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: A schematic representation of the components of a powder-based 3D printer in an oblique sectional view.

FIG. 2: Sequence of a conventional 3D printing process using layer-wise radiation curing.

FIG. 3: Sequence of a construction process comprising radiation curing which is not effected in every layer.

FIG. 4: A schematic representation of the application of binder (400) onto the supplied particulate material, wherein (401) represents one particle and the dark arrow indicates the direction of penetration. In this case, the strength of the component is obtained by the binder bonding the particles of the particulate material and curing of the binder taking place, which results in a solid connection of the binder with the particulate material. FIGS. 4a-4d describe the sequence of the binder penetrating into the particulate material.

FIG. 5: FIGS. 5a and 5b show the energy input/heat input (500, 501, 502) and the energy loss/heat loss (503, 504, 505), respectively. FIG. 5c shows very inhomogeneous temperature distribution, and in this case, the isothermal lines (510-514) within which a solidification reaction is bound to happen, are shifted far towards the centre of the construction container. This considerably reduces the useful construction space, and the economic viability of the device and the method is disadvantageous. FIG. 5c shows the situation in the disclosed method, and it becomes evident that the useful construction space or the useful applied powder cake, respectively, is much greater which results in increased economic viability of the machine, and at the same time, the properties of the component may be advantageous. This is shown by isothermal line 510, which includes a large area that is kept homogeneous in its temperature during the process and thus yields positive process results.

FIG. 6: Preparation of the prepolymer as exemplified by a resol

FIG. 7: Condensation reaction of resol to cross-linked resite

DETAILED DESCRIPTION

In the following, several terms will be defined more precisely. Otherwise, the terms used shall have the meanings known to the person skilled in the art.

In the sense of the invention, “3D printing methods” are all methods known from the prior art which enable the construction of components in three-dimensional moulds and are compatible with the described process components and devices. In particular, these include powder-based methods, containing as one ingredient aqueous solutions or/and other fluid components or solvents which have to be extracted from the moulded part or escape from the moulded part during or for its solidification. The solidification and quality of the moulded part can be selectively influenced by the invention, with other quality features remaining unchanged or even being positively influenced.

A “moulded part”, “model”, “3D moulded part” or “component” in the sense of the invention means all three-dimensional objects manufactured by means of the method according to the invention or/and the device according to the invention which exhibit dimensional stability.

The “device” used for carrying out the method according to the invention may be any known 3D-printing device which includes the required parts. Common components include a coater, a construction field, means for moving the construction field or other components, a dosage device, a print head, a heating medium, displacement means for batch-wise or continuous processes, and other components which are known to the person skilled in the art and will therefore not be described in detail herein.

A “construction platform” or “construction area” moves, according to the disclosure, with respect to the printing and coater level. This relative movement takes place during the construction process in interrupted movements in layer thickness. The movement defines the layer thickness. As an alternative, the device may be configured such that the other parts of the device move upwards, thereby adjusting the layer strength or layer thickness.

A “construction container” or “job box” provides a construction space. Accordingly, it has a bottom, walls and an open access area, the construction space. The construction container always comprises parts which do not move relative to the frame of the 3D printing device. Exchangeable construction containers, known as job boxes, allow virtually constant operation of the machine, because the job boxes can be moved in and out of the machine. The parts of a first construction operation can thus be unpacked outside the device (3D printing device), while new parts can already be printed in a second construction container within the machine.

A “construction space” in the sense of the invention is the geometric location where the particulate material bed grows during the construction process by repeated coating with particulate material. The construction space is generally bounded by a bottom, i.e. the construction platform, by walls and an open top surface, i.e. the construction plane. The construction plane may be horizontal, but may also form an angle, for example, in continuous processes, so that coating is performed obliquely, at an angle.

The “particle materials” or “construction materials” “or particulate material” of use herein may be any materials known for powder-based 3D printing, in particular sands, ceramic powders, metal powders, plastic materials, wood particles, fibre materials, celluloses or/and lactose powders. The particulate material is preferably a free-flowing powder when dry, but a cohesive, cut-resistant powder may also be used. In a preferred aspect, the particulate material used may be considered a “passive powder material”, because it is not directly involved in the binding reaction, i.e. solidification, of the component, but is merely solidified or “connected” by the binder system into a solid component. It may display inert behaviour. The applied particulate material may also be referred to as powder cake.

“Adjusting the temperature” or “tempering” means that a specific temperature is adjusted in the construction space or/and the applied particulate material or that the construction space is adjusted to a selected temperature. In one aspect, the applied particulate material is tempered, in particular, and the temperature is maintained, for example, at approx. 60, 70, 80, 90, 100, 110, 120, 130, 140, 150° C., or 80 to 100° C.

A “binder” or “binder system” is the material which is selectively applied onto the particulate material by means of the print head and which leads to solidification and, thus, the production of the component. The binder system comprises a solvent and further components, e.g. monomers, oligomers and/or polymers. The binding mechanism is a polymerization reaction. This results in a solid material which is capable of binding the particles in the powder. As the basic material, a pre-polymeric phenolic resin is preferred.

The “diffusion length” corresponds to the expansion of an applied binder in the particulate material and is influenced, inter alia, by the volume, temperature and composition of the binder.

Preferred embodiments will be described below.

What is disclosed is a method for the layered construction of components, wherein a particulate material is applied onto a construction area in a construction space layer by layer with a layer thickness, a binder is selectively applied, the temperature in the construction space or/and in the particulate material applied is adjusted to a desired temperature, and the steps of material application and binder application are repeated until a desired component is obtained, wherein the temperature in the construction space or/and in the material applied is adjusted to at least 70° C. and maintained for at least 2 hours, with the areas onto which the binder was selectively applied solidifying and forming the component.

Using the method disclosed herein, components are produced, on the one hand, which exhibit positive material properties, and on the other hand, the disclosed method allows the effective space within the applied powder cake to be increased, because a constant temperature, necessary for the duration required for the reaction, can be adjusted and maintained.

The temperature is selected such that the reaction temperature required for the solidification with positive component properties is set to be homogeneous in the powder cake. Preferably, the temperature is adjusted to 70 to 90° C., preferably at least 80° C., more preferably at least 90° C., still more preferably 80 to 150° C., and even more preferably 80 to 100° C. This is also referred to as an isothermal line, and an isothermal line of 80° C., preferably 90° C., is preferred.

In this manner, a substantially homogeneous temperature is adjusted and maintained in an advantageously large area of the powder cake, i.e. in an area of the applied particulate material.

In the method, the temperature is maintained over a period required for the reaction, preferably for 3 to 10 hours, more preferably for 4 to 6 hours, and still more preferably for at least 4 hours.

Use can be made of any suitable particulate materials known to the person skilled in the art, the particulate material preferably being a plastic material, a sand, a ceramic material or a metal. In this case, the particle size may be selected as required in conjunction with the other process parameters. Preferably, the average grain size is at least 8 μm, more preferably 10 μm to 1 mm.

The particulate material may be applied in different layer thicknesses, with a layer thickness of 50 to 800 μm being preferred.

The binder is adapted to the other process materials and conditions, and the binder used may preferably be a binder system comprising monomers, oligomers or/and polymers and a solvent, said solvent preferably being an aqueous or alcoholic solvent.

The component obtained by the method preferably has a green strength in the component of at least 280 N/cm².

The method allows a final strength (bending strength) in the component of at least 300, preferably at least 500 N/cm², to be achieved either directly or after further process steps.

In the method, the process conditions are set such that the component produced thereby has a loss on ignition of less than 3%, preferably less than 2.5%, more preferably less than 2.2%.

Further process steps are possible; the resulting component can be subjected to further processing steps.

In another aspect, the disclosure relates to a binder system comprising monomers, oligomers or/and polymers and a solvent, said solvent preferably being an aqueous or alcoholic solvent. The binder system preferably comprises a pre-polymeric phenolic resin.

In another aspect, the disclosure relates to a material system comprising a particulate material as described above and a binder system as described above.

In another aspect, the disclosure relates to a device for producing a component, said device comprising a construction space with a construction platform, means for applying a particulate material, means for selectively applying a binder system, means for adjusting a temperature in the construction space or/and the particulate material. Further aspects of the invention will be presented in more detail below, and these device elements can be combined with each other in any useful and functional manner desired.

Finally, in another aspect, the disclosure relates to a solid body (component) produced by means of a method, a binder system, by means of a material system or/and a device as described herein, wherein the solid body preferably has a bending strength of 500 N/cm².

Further aspects will be described below.

One aspect of the method is a binding agent system or binder (400), which is printed on a powder (401) that is neutral with regard to the reaction and cures at a substantially higher temperature than room temperature over several hours. The majority of said curing takes place during the construction process. In this case, the entire resulting powder cake is kept warm for hours.

Various particulate materials (401) can be used as the powder. This includes ceramic powders, sand or even metal powders. For the method, the powder grains (401) should not be substantially smaller than 10 μm. Particles (401) greater than 1 mm generally make safe processing difficult. These statements refer to the average grain size. However, considerable parts of the aforementioned maximum and minimum grains (401) are detrimental to the process even if the average grain size requirements are met.

The particles are processed in the device into a thin layer (107) by a coater (101) in conjunction with the construction platform (102). For this purpose, the particulate material (401) is supplied at or from a starting position and smoothed by the coater (101) moving over the construction field. The respective position of the construction platform determines the layer thickness.

In a resin system or binder (400) according to the invention, the powder cake is kept at a temperature of 80° C. for at least 4 hours. This results in a bending strength in the components of over 300 N/cm² with a loss on ignition of less than 2.2%.

The binder system (400) includes monomers, oligomers and/or polymers as binding ingredients. These are solved in a solvent. The binding mechanism is a polymerization reaction. It results in a solid material which is capable of binding the particles in the powder. As the basic material, a pre-polymeric phenolic resin is preferred.

According to the invention, the binder system (400) is configured for use in inkjet print heads (100) comprising piezo elements. In this case, its viscosity ranges from 5 to 20 mPas. Steam pressure is less than 3,000 Pa at room temperature. Surface tension is in the range of from 30-50 mN/m. The binder system is adjusted such that it takes at least approx. 1 minute for the reversible drying-up of the print head to hinder the function of first jets.

The binder system (400) exhibits extremely low reactivity at room temperature. This protects the print head (100), which remains highly reliable even after a long service life. Dried binder is easy to remove at room temperature even after weeks. This facilitates both cleaning of the device and reactivation of dried-up jets.

In the case of phenolic resins, the solvent for the binder system may be water. Therefore, the binder system can be regarded as hardly noxious if handled properly.

Due to the heating effect of the hot construction field surface, the print head (100) is cooled actively or passively, allowing the drop mass and, thus, the input to be kept constant throughout the construction process. Passive cooling may be effected by contact with the print head cleaning unit. Active cooling may be achieved, for example, by a cooling element through which cooling water flows and which is mounted to the print head (100). The introduction of pre-cooled compressed air, and also a fan, are suitable for cooling.

Upon reaching the surface of the particulate material, the binder printed by the print head (100) penetrates slowly into the powder cake as a function of the surface tension. In this case, a certain diffusion length is desired. Said diffusion is necessary to bond the individual layers with each other. The diffusion length depends on the fluid parameters, but also on the temperature on the construction field. Moreover, the temperature is controlled such during construction that the printed layers cure slowly, thus allowing interlaminar bonding.

For example, if water is used as the solvent, it is advantageous to work below 100° C. Above this limit, evaporation effects occur which may adversely affect the surface quality of the components.

It is advantageous for the process to set a diffusion length of 1.5 times the layer thickness. This results in a good compromise between anisotropy in the direction of construction, resolution and quality of the bottom surfaces of the component.

In this process, layer thicknesses of 50-800 μm are possible and useful. They are adjusted according to the powder material and the desired construction progress.

The device comprises means (200, 300-304) for heating the powder cake and keeping it warm. In this case, the energy input can be partly insulated by the powder, so that powder once heated up cannot cool off rapidly.

Infrared heat sources can be used as the heating means. They may be arranged statically (302) above the construction field or moved (200) over the construction field by moving parts of the device. Halogen radiators made of quartz glass as well as ceramic radiators are suitable. Mirrors for IR radiation are also suitable to influence and control the heat balance.

For example, an IR radiator with a maximum power of 9.5 kW and a length of one metre can be used to heat a construction field of 100×60 cm. In this case, the radiator is moved back and forth over the construction field, for example, at a speed of 0.05 m/s. This process is usually combined with the coating process. For a coating time of approx. 60 sec., which results substantially from printing and irradiation, over 90° C. may be reached in the powder cake during the construction process. As an alternative, a separate irradiation passage carried out x times every n layers may be used in addition to the described irradiation and/or as the only heating routine, with n≥2, preferably every 2 to 5 layers, particularly preferably every 3 layers, and with x≥1 irradiation, preferably 2 to 5, particularly preferably 3 to 4 irradiations.

Also, hot air may sweep (301) over the construction field and thereby heat it up. All forms of hot air blowers (301) are suitable for this purpose. Direct preheating of the powder with an air stream or a resistance heating is also possible. Also, a contacting, heated metal may be guided over the powder (304) to heat it up.

Another suitable process means are heatings in the construction container wall (300) and/or in the construction platform (301). On the one hand, these can introduce heat to the process; on the other hand, by active insulation, they can reduce heat losses.

Such heatings may be provided, for example, as electric resistance heatings. They can be controlled via standard control devices. For this purpose, sheets are commonly used, for example, which can be glued onto metallic surfaces, such as the construction container walls. The same effect is provided by heating cartridges which are inserted into bores in metallic plates.

In the case of poor insulation, power outputs of up to 5 W/cm² are required for temperatures up to approx. 90° C. Depending on the insulation quality, enormous amounts of energy can be saved here.

Further, active insulation can be carried out using a heat transfer medium. The heat transfer medium, for example water or oil, may be transported in tubes, which are usually made of copper, extend within the job box wall and the job box floor in a contacting manner and preferably extend in a meandering manner so as to achieve the most uniform heating of the box possible.

Passive insulations are also useful for temperature control in the construction container. In this case, different “materials”, such as those also common in the construction industry, can be used: Mineral insulations, plastic materials, foams, but also air, vacuum etc. The passive insulations are installed such that the heat flow from the construction container is reduced.

Another essential means of the device is its control unit. The control unit determines the temperature control within the device. For this purpose, means for detecting important temperature parameters may be present in the device. This allows control circuits to be provided. A simple variant is the control of the power input into the device. In this case, a higher power input takes place during the warm-up phase of the machine. The power is gradually reduced during the ongoing production process.

In the above-described device with a 9.5 kW radiator, for example, the power is reduced from 100% to 70% during the first 3 hours of the construction process. With this power, the entire process is carried out after reaching stationary conditions.

The combination of heating elements and insulation must be adapted so as to achieve an homogeneous temperature field (e.g. FIG. 5d) in the powder cake. The required homogeneity depends on the binder system. A certain threshold must definitely be exceeded to ensure reliable curing of the binding agent.

In general, the selected temperature must not be too high, because otherwise the printing and construction process will be affected. The construction field surface temperature should not exceed 90° C. here. In order to achieve sufficient strength, the temperature must be above 80° C. In other words, the temperature has to be within a 10K range.

If the temperature distribution becomes very inhomogeneous (FIG. 5c), the isothermal lines (510-514) within which a solidification reaction is bound to happen, are shifted far towards the centre of the construction container. This considerably reduces the useful construction space.

The binding agent system is a heat-curable pre-polymeric resol resin solution and/or novolac solution, which reacts at the already described conditions, partly or fully during the construction process into an insoluble and infusible resite. The temperature control throughout the construction process is selected such that curing does not take place ad hoc, but over a period of several hours, allowing interlaminar bonding. (FIG. 7)

Resols and novolacs are prepolymers of a phenol-formaldehyde co-condensate. Resols are prepolymers which are polymerized with phenol under alkaline conditions with an excess of formaldehyde. (FIG. 6)

Resols include an increased number of free hydroxyl groups which tend to condensate further under temperature influence and form an insoluble resite.

Novolacs are prepolymers synthesized under acidic conditions with a shortage of formaldehyde and co-condensed with resol to resite under temperature influence or polymerized with addition of a formaldehyde donor (e.g. urotropine) and temperature increase.

Due to its production process, the prepolymer contains residual monomers of phenol and formaldehyde. The phenol content is preferably below 5% and particularly preferably below 1%.

The formaldehyde content is preferably below 0.3%, particularly preferably below 0.1%.

By adding a solvent, which consists of mono- and/or polyalcohols and/or water or exclusively of water, the binder is diluted or adjusted, respectively, to such an extent that the viscosity is in the range of 5-20 mPas, preferably between 5-10 mPas and particularly preferably between 5-8 mPas.

For viscosity fine adjustment, further modifiers may be used, such as polyvalent alcohols like glycol, propanediol or propylene glycols, further carboxymethylcelluloses, xylitol, sorbite or gum arabic. Preferably, 1%-9%, particularly preferably 3%-7%, are used.

For surface tension adjustment, common surfactants, such as sodium lauryl sulfate or sodium laureth sulfate as well as fluorine-containing and silicone-based surfactants, may be used.

The binder liquid usually consists of 30%-40% prepolymer, 60%-70% solvent and 1%-7% viscosity modifier.

The binder input can be adjusted over a wide range according to the desired final strength and loss on ignition. Usually, the input and temperature control are selected such that the green strength is not below 280 N/cm² to ensure safe handling of the components. If the condensation reaction is not fully achieved during the construction process, a subsequent oven process may complete the reaction. Preferably, the components are subsequently baked for 1-4 h at 120° C.-150° C., particularly preferably 2-3 h at 130° C.−140° C. Any solvent still remaining will also be completely expelled by this operation.

Usually, inputs of 5%-8% by weight of the particulate material are used, which make a sufficient green strength and a high final strength accessible.

The examples describe preferred embodiments, without being construed as limiting.

EXAMPLES

Example 1

Binder mixture: Resol/novolac-prepolymer 35%, water/i-propanol (80/20) 63%, 1,2-propanediol 3%

Input: 5.8% by weight

Particulate material: AFS 100 silica sand

Green strength: 380N/cm²

Final strength: 540N/cm² (after 3 h @ 135° C.)

Loss on ignition: 2%

LIST OF REFERENCE NUMERALS

• • 100 inkjet print head
  • 101 powder coater
  • 102 construction platform
  • 103 component
  • 104 construction field edge
  • 107 powder layers
  • 200 heat source
  • 300 Resistance heating/liquid heating/air heating for the container wall
  • 301 Resistance heating/liquid heating/air heating for the construction platform
  • 302 hot air blower
  • 303 static heaters
  • 304 Resistance heating with contacting element
  • 400 Binder/binding agent system
  • 401 Powder particles
  • 500 Energy input onto the construction field surface
  • 501 Energy input into the powder cake through the construction container wall
  • 502 Energy input into the powder cake through the construction platform
  • 503 Energy loss from the construction field surface
  • 504 Energy loss of the powder cake through the construction container wall
  • 505 Energy loss of the powder cake through the construction platform
  • 510 Isothermal line in the powder cake for 90° C.
  • 511 Isothermal line in the powder cake for 80° C.
  • 512 Isothermal line in the powder cake for 70° C.
  • 513 Isothermal line in the powder cake for 60° C.

Claims

1. A method for the layered construction of components, wherein a particulate material is applied onto a construction area in a construction space layer by layer with a layer thickness, a binder is selectively applied, a temperature in the construction space or/and in the particulate material applied is adjusted to at least 80° C., and the steps of material application and binder application are repeated until a desired component is obtained, wherein the temperature is maintained for at least 2 hours, with the areas onto which the binder was selectively applied solidifying and forming the components.

2. The method of claim 1, wherein the binder used is a binder system comprising: i) one or more monomers,ii) one or more oligomers or polymers, andiii) a solvent.

3. The method of claim 2, wherein the solvent is an aqueous or alcoholic solvent.

4. The method of claim 1, wherein the component thus obtained is subjected to further processing steps.

5. The method of claim 1, wherein the temperature is adjusted to 80 to 150° C.

6. The method of claim 5, wherein a substantially homogeneous temperature is adjusted and maintained in an area of the applied particulate material.

7. The method of claim 6, wherein the temperature is maintained for 3 to 10 hours.

8. The method of claim 7, wherein the particulate material is a plastic material, a sand, a ceramic material or a metal.

9. The method of claim 8, wherein the particulate material has an average grain size of at least 8 μm.

10. The method of claim 9, wherein the layer thickness is 50 to 800 μm.

11. The method of claim 1, wherein the particulate material consists of particles.

12. A method for the layered construction of components, wherein a particulate material consisting of particles is applied onto a construction area in a construction space layer by layer with a layer thickness, a binder is selectively applied, the temperature in the construction space or/and in the particulate material applied is adjusted to a desired temperature, and the steps of material application and binder application are repeated until a desired component is obtained, wherein the temperature in the construction space or in the material applied is adjusted to at least 70° C. and maintained for at least 2 hours, with the areas onto which the binder was selectively applied solidifying and forming the components.

13. The method of claim 12, wherein the binder used is a binder system comprising: i) one or more monomers,ii) one or more oligomers or polymers, andiii) a solvent.

14. The method of claim 12, wherein a homogeneous temperature is adjusted and maintained in an area of the applied particulate material; the homogenous temperature is maintained for 3 to 10 hours;the particulate material has an average grain size of at least 8 μm,the layer thickness is 50 to 800 μm; andthe binder includes an aqueous solvent.

15. The method of claim 12, wherein the particulate material is a plastic material.

16. The method of claim 12, wherein the particulate material is a sand, a ceramic material or a metal.

17. The method of claim 12, wherein the binder includes a phenol-formaldehyde co-condensate having below 5% residual phenol and below 0.3% residual formaldehyde.