ES2915589B2 - Composite material for use in stereolithography and obtaining procedure - Google Patents

Description

DESCRIPTION

COMPOSITE MATERIAL FOR USE IN STEREOLITHOGRAPHY AND OBTAINING PROCEDURE

TECHNIQUE SECTOR

The invention belongs to the field of additive manufacturing technology (B33Y) and specifically to the sector of materials specially designed for additive manufacturing (B33Y 50/00). It refers to the use of cork powder of different granulometries and in different percentages by weight with respect to a polymer matrix in order to add a photocurable resin and use it in 3D printing techniques by tank photopolymerization or stereolithography; specifically by the technique of stereolithography or SLA.

BACKGROUND OF THE INVENTION

Cork is a material of natural origin, product of the formation of the bark of the cork oak ( Quercus Suber). Due to its unique cellular structure, with cells between 10 ^m and 70 ^m in size, filled with a gas similar to air, it has unique physicochemical characteristics: it is a good thermal and acoustic insulator, waterproof, It is a light, low-density, compressible, elastic and flexible material. Its Poisson's ratio is 0, this means that when the volume of the cork is compressed in one direction, no deformation occurs in the perpendicular direction. This characteristic makes cork a material that, when used in extreme conditions, resists the stress to which it is subjected. These properties justify that today it is a widely used material for countless applications both in household items, sports equipment and construction as well as in technological industries such as aerospace, naval or automotive. In the last decade, the search for sustainable materials of natural origin has joined the growing need to find new techniques for the technological modernization of the industry. That is why the so-called fourth industrial revolution or Industry 4.0 arose in this period, which encompasses a series of techniques such as artificial vision, robotics, big data, or additive manufacturing (AM), popularly known as 3D printing(1 ). Cork has emerged as an ideal material for this fusion between sustainability and innovation. This is justified, on the one hand, by the immense amount of cork dust generated by the cork industry, which is treated as waste or used as biomass in energy generation, and on the other, by the aforementioned magnificent properties of cork as a material . This intense search for innovation favors the emergence of multidisciplinary fields within materials science, with research into new materials for 3D printing being a field with intense research activity in recent years(2).

Within the FA techniques we focus on stereolithography or SLA. This technique was developed in the 1980s by 3D Systems (Valencia, CA, USA)(3) and is a tank photopolymerization technique, in which a photocurable liquid resin is deposited and where a polymerization reaction occurs when applying radiation (ultraviolet laser, UV) with a certain wavelength. It is currently a technique that is used in various fields, such as naval (4), standing out especially in medicine, specifically in tissue engineering or bioprinting (5) and in other fields of design, such as personalized jewelry or in realization of pre-commercial prototypes.

One of the main drawbacks of this technique is the low mechanical properties of photosensitive resins, the improvement of the properties of photocurable resins being studied. The typical composition of photocurable resins by UV radiation is made up of oligomers, monomers that act as diluents and photopolymerization initiators, among other excipients. To a large extent, commercial and synthetic photocurable resins are based on acrylic compounds that act as free radicals (acrylates)(6). It is well documented in the scientific literature that additives can modify the mechanical properties of resins such as Young's modulus, flexural strength, toughness or hardness(7). That is why the nature of the additive used to modify the photosensitive resin plays a decisive role in modifying these properties, as well as the shape and size of the additive used(8)(9).

Currently there are several studies that use powdered cork as an additive to a thermoplastic base for additive manufacturing, for the FA technique of fused deposition modeling or FDM (fused deposition modeling), also known as FFF (fused filament). manufacturing). However, there are no studies or any reports that report the use of cork powder added to photosensitive resins for stereolithography or SLA; This is probably due to the fact that the opaque nature of the cork has led us to believe that its addition to a photosensitive resin would cause it to stop being photosensitive and would prevent its photocuring by hindering the absorption of the UV radiation applied during the process. In this invention it is shown that it is possible to do so.

Among the studies published in specialized indexed journals on the addition of cork in thermoplastics for FFF, the recently published by SP Magalhaes da Silva et. to the. ( 2020) ( 10), where they use cork to add to a biodegradable thermoplastic such as polylactic acid (PLA) and manufacture a filament that can be used in 3D printing. In this sense, we highlight the works in the same line of Fugen Daver et. al ( 2018) ( 11\ where the authors have developed PLA/cork filaments, the work of N. Gama et al. (2019)(12), related to PTU/cork filaments, and the work of F. Brites et al. .al, where he studies the optimization of the use of cork for filaments for 3D printing ( F. Brites et.al (2017)(13)) and polyethylene (PE) composites added with cork for large-format 3D printing (F Brites et.al (2019)(14)) There are also other studies related to the production of composite materials and cork, such as those included in the article by Emanuel M. Fernández et.al.(2014)(15 ), in which the processability parameters and properties of polypropylene composite materials with cork prepared by the extrusion technique and intended this time for injection, not 3D printing techniques, are studied.

The patent literature shows that there are no relevant documents related to the present invention. Some documents are cited that show the current state of the art:

- WO2020037279A1 describes the manufacture of a protective helmet from stereolithography from a photocurable liquid polyurethane resin and where cork is mentioned as a possible energy attenuation additive.

- US2017234268A1 refers to the manufacture of a hybrid rocket motor from stereolithography and where graphite, silicon oxide, fiberglass or cork are used as additives.

- JP2016037571A discloses a composition of a polyolefin resin with polylactic acid and cork powder (as an additive), which can be manufactured using a 3D printer. However, it is not disclosed that it can be manufactured by stereolithography.

Thus, the previously described state of the art, both based on the cited scientific literature and on the patents, makes it possible to confirm that there is no reference in which cork powder is used in the case of stereolithography and, therefore, neither is the size of the particles specified nor the specific percentage by weight in the mixtures.

List of cited documents:

patents

WO2020037279A1. Bologna, Vittorio; Gillogly, Murphy; Ide, Thad M. System and method for designing and manufacturing a protective helmet. Publication date 2020 02-20. Riddell, INC.

US2017234268A1. Danforth Jeremy C.; Summers Matt H. hybrid rocket motor with integral oxidizer tank. Publication date 2017-08-17. Raytheon Company JP2016037571A Kentaro Kanae, Minoru Maeda, Shohei Sasama. Resin composition for molding and filament for molding. Publication date 2016-03-22.

Articles

1. Haleem, A., Javaid M. Additive Manufacturing Applications in Industry 4.0: A Review. J Ind Integr Manag. 2019;4.

2. Tofail SAM, Koumoulos EP, Bandyopadhyay A, Bose S, O'Donoghue L, Charitidis C. Additive manufacturing: scientific and technological challenges, market uptake and opportunities. Mater Today. 2018;21(1):22-37.

3. Salonitis K. 10.03 - Stereolithography. In: Hashmi S, Batalha GF, Tyne CJ Van, Yilbas B, editors. Comprehensive Materials Processing. Oxford: Elsevier; 2014. p. 19-67.

4. Phillips BT, Allder J, Bolan G, Nagle RS, Redington A, Hellebrekers T, et al.

Additive manufacturing aboard a moving vessel at sea using passively stabilized stereolithography (SLA) 3D printing. Addit Manuf. 2020;31:100969.

. Ng WL, Lee JM, Zhou M, Chen Y-W, Lee K-XA, Yeong WY, et al. Vat polymerization-based bioprinting{\textemdash}process, materials, applications and regulatory challenges. Biofabrication. 2020;12(2):22001.

. Ligon-Auer SC, Schwentenwein M, Gorsche C, Stampfl J, Liska R. Toughening of photo-curable polymer networks: A review. Polym Chem. 2016;7(2):257-86. . Dizon JRC, Espera AH, Chen Q, Advincula RC. Mechanical characterization of 3D-printed polymers. Addit Manuf. 2018;20:44-67.

. Medellin A, Du W, Miao G, Zou J, Pei Z, Ma C. Vat Photopolymerization 3D Printing of Nanocomposites: A Literature Review. J Micro Nano-Manufacturing.

2019;7(3).

. Liu Y, Lin Y, Jiao T, Lu G, Liu J. Photocurable modification of inorganic fillers and their application in photopolymers for 3D printing. Polym Chem.

2019;10(46):6350-9.

0. Magalhaes da Silva SP, Antunes T, Costa ME V, Oliveira JM. Cork-like filaments for Additive Manufacturing. Addit Manuf. 2020;34:101229.

1. Daver F, Lee KPM, Brandt M, Shanks R. Cork-PLA composite filaments for fused deposition modelling. Compos Sci Technol. 2018;168(October):230-7.

2. Gama N, Ferreira A, Barros-Timmons A. 3D printed cork/polyurethane composite foams. Mater Des. 2019;179:107905.

3. Brites F, Malga C, Gaspar F, Horta JF, Franco MC, Biscaia S, et al. Cork Plastic Composite Optimization for 3D Printing Applications. Proceeded Manuf.

2017;12(December 2016):156-65.

4. Brites F, Malga C, Gaspar F, Horta JF, Franco MC, Biscaia S, et al. The Use of Polypropylene and High-Density Polyethylene on Cork Plastic Composites for Large Scale 3D Printing. Appl Mech Mater. 2019;890:205-25.

5. Fernandes EM, Correlo VM, Mano JF, Reis RL. Polypropylene-based corkpolymer composites: Processing parameters and properties. Compos Part B Eng. 2014;66:210-23.

EXPLANATION OF THE INVENTION

The present invention is based on the additivation of finely divided powdered cork to a photocurable resin, forming a composite material that can be used as a starting material to produce AM using the stereolithography technique.

As will be explained later, in the formulation of the product object of the invention, the size of the cork powder must be below 150 pm and the percentage of cork powder used in the product object of the present invention with respect to the matrix of photocurable resin must be less than or equal to 30% (p/p), since it has been observed that percentages higher than those mentioned have a negative effect on the processability of the composite material.

The procedure for obtaining the product object of the invention begins with the grinding of previously cooked virgin cork until obtaining a fine cork powder; The starting cork can also come from waste from companies that are dedicated to the transformation of cork. Next, the cork dust is screened, separating it by size in microns. This screening can be carried out with a manual procedure, using a manual sieve with a suitable mesh size for the desired size or with the help of a mechanical sieve with several sieves arranged in a vertical position, with three-dimensional shaking, whose size in microns of the light mesh is available from largest to smallest size for convenience. This sieving process can also be carried out with other types of sieving machines. Next, by manual or mechanical mixing (helical agitator), with a photocurable resin, a composite material is obtained where the percentage by weight and size of the cork powder is variable according to interest. For the product object of the invention, the size of the cork powder must be below 150 pm. For larger sizes, a considerable deterioration in the processability of the material is observed. For this, a detailed statistical study is carried out from the images obtained by scanning electron microscopy (SEM) (figure 1 and figures 2A and 2B). The composite material obtained is introduced into an ultrasonic bath to extract the air introduced during its agitation and give the compound stability. The minimum treatment time will be 10 minutes, followed by the extraction of air in a vacuum filtration desiccator. The obtained composite material is conveniently deposited in a photopolymerization tank for stereolithography. After the 3D printing process, parts are obtained where the integration of the cork with the polymeric matrix is evident (figure 3).

Subsequently, these parts can optionally be subjected to a UV curing process in order to modify their mechanical properties compared to uncured parts, according to the desired final application of the product, as is usually done in additive manufacturing techniques based on stereolithography. These differences in the mechanical properties before and after the curing treatment are shown in figure 4. The differences between the properties studied, Young's modulus (E) (figure 4A), elongation at break (%8) (figure 4B) and tensile strength (a) (figure 4C), are discussed below. For each test, five test tubes of each cork powder photocurable resin formulation were manufactured using 3D printing stereolithography, considering the average for the calculation.

Regarding the Young's modulus, the initial loss of the modulus for uncured samples is recovered after the curing process, and is even exceeded when the granulometry decreases to a size below 45 pm. Depending on the granulometry size used, this property can be modulated with respect to the cured resin, even exceeding the initial value of the photocurable resin.

In the elongation at break, a property related to the ductility of the material, the uncured specimens present greater ductility compared to the cured ones, which can be improved when the cork is better integrated into the photocurable matrix; this is achieved by reducing the size of the cork dust particle. If the application requires a light and ductile material, a smaller granulometry will be used without a curing process. Otherwise, ductility is sacrificed with a UV curing treatment.

Regarding tensile strength, when adding the photocurable resin, the tensile strength decreases again. Different granulometries of cork powder provide different tensile strength values. By subjecting the composite material to a UV curing treatment, in composite materials with smaller sizes in the cork powder granulometry, where it integrates more effectively into the photocurable resin matrix, this resistance is increased by a large percentage. to tensile strength, as was the case with Young's modulus.

Regarding the thermal conductivity (k), it decreases as the granulometry of the cork powder in the composite material is reduced (Figure 5). Measurements shown are for already cured composites, where the difference from uncured is not significant when comparing uncured and cured composites. In agreement with the analysis of the mechanical properties, the effect in reducing the size of the cork powder benefits the integration in the photocurable resin, this time decreasing the thermal conductivity. Modifying the size of the cork powder in the composite material modulates this property, making it possible to create low-density thermal protection materials.

As can be seen from the above discussion, the present invention enables the creation of low-density composite materials with significant differences in mechanical and thermal properties. Precisely the introduction of finely divided cork powder causes a decrease in the density of the composite material with respect to the matrix used. Given the variety of applications in which cork can be used due to its properties, the latter can be modulated by varying the size and percentage by weight of the cork powder. The percentage of cork powder used in the product object of the present invention with respect to the photocurable resin matrix is less than or equal to 30% (p/p), since it has been observed that higher percentages have a negative effect on processability. of the composite material.

With this procedure, the way is opened for rapid prototyping by means of stereolithography of parts based on composite materials of cork and photocurable resin and for their use in the aerospace, naval and automobile industries, among other applications, where this material is already used for purposes varied such as thermal protection, acoustic, anti-vibration and decoration as main applications. Cork is also a material of 100% natural origin whose unique physical-chemical characteristics mean that technological applications using this technique are varied.

Additivation of cork to photopolymerizable resins, before carrying out the present invention, had not been attempted because it might seem a failed approach, since it could be expected that it would give rise to an opaque composite material, which could not undergo the photopolymerization process, necessary for techniques work 3D photopolymerization in tank. In this invention we show that it is possible to carry out the process for certain size distributions and concentrations of cork powders, in specific photopolymerizable resins used in stereolithography.

BRIEF DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, a set of images is attached as an integral part of said description, illustrating the explanation of the invention with an illustrative and non-limiting nature. .

Figure 1. SEM images of cork powder without screening (left) and cork screened using a mesh size of 45 pm (right), both used in the described technique with satisfactory results.

Figure 2. Statistical study of the particle size distribution used in this technique: A) Unscreened cork powder, B) Screened cork powder using a mesh size of less than 45 pm. The lognormal distribution function is shown.

Figure 3. Integration of cork powder (5% w/w) in the photocurable resin after 3D printing by stereolithography. Scanning Electron Microscopy (SEM) images. A) Unscreened powder photocurable resin. B) Photocurable resin cork dust less than 100 pm and greater than 45 pm. C) Photocurable resin cork dust less than 45 pm. D) Surface of the cork powder photocurable resin piece.

Figure 4. Comparative graphs between the mechanical properties of the uncured material and material cured with UV radiation. Young's modulus (A), percent elongation at break (B) and tensile strength (C) are shown for the following samples tested: Pristine resin (CR) and pristine resin-cork composite (CCR) of different Granulometry expressed in micrometers: mixture granulometry (mix), 150 pm<powder>100 pm, 100 pm <powder>75 pm, 75 pm <powder>45 pm and dust <45 pm.

Figure 5. Graph of thermal conductivity values of different composite materials with differentiated granulometry: 150 pm<powder>100 pm, 100 pm <powder>75 pm, 75 pm <powder>45 pm and dust <45 pm.

PREFERRED EMBODIMENT OF THE INVENTION

The invention consists of the formulation of a composite material, based on a photocurable resin to which finely divided cork is added. The peculiarity of this invention resides in the nature of the additive. Cork is a natural biopolymer that is opaque to the naked eye. When adding photocurable resins, one of the main problems is that the dispersed particles inside the photoresist can scatter UV light, making it difficult to process the composite material. In the present invention, this problem is not present at a level that prevents the photosensitive resin, after being additivated with cork powder, from being used in additive manufacturing by stereolithography.

A second aspect of the invention refers to a procedure for obtaining the product object of the invention.

An embodiment of the invention is briefly and concisely explained below, by way of example, merely for illustrative purposes and without being construed as limiting.

EXAMPLE 1 . In this example, the basic principle of the technique for obtaining the product object of the invention is exposed, so that an expert in the field of development of composite materials for 3D printing techniques can carry it out and understand the essentials of the developed invention.

The stages of the procedure included in this example are:

1. Cork grinding from 100% natural cork. The grinding was carried out with a mechanical procedure in a mill or by sanding it and dust aspiration to obtain a fine cork powder. The mean size of this powder was 73.5 pm, as seen from the statistical study of the particle size distribution in Figure 2A.

. Cork dust screening. A cork dust fraction of less than 45 pm was obtained by mechanical screening, with a mean particle size of 29.3 pm (figure 2B). Likewise, other sizes of cork powder were used to demonstrate the feasibility of the technique in the range of cork powder sizes shown in figure 2A. The nomenclature for the samples that illustrate this example is as follows: 5%CCR ( N/S)CXX where 5%C expresses the amount of cork powder by weight with respect to the photocurable resin, CR means clean resin (pristine resin) , NC: not cured, SC: yes cured, depending on the sample, and XX indicates the size in microns below which the cork used to make the composite material is. For composite material made from unscreened powder, the word "mix" appears instead of a number.

. Mixture of the sieved cork powder with the photocurable resin. Mechanical mixing was carried out with a helical stirrer with an anchor-type blade. 100 grams of composite material were prepared, where the cork powder, previously described in point 2, was added in a proportion of 5% (w/w) with respect to the resin, that is, 95 grams of photocurable resin weighed on a scale analysis and 5 grams of cork powder weighed under the same conditions. The resin used in this example was a commercial Form Clear v2 resin, a mixture of a photoinitiator and acrylic monomers and oligomers, supplied by Formlabs. The mixture obtained was introduced into an ultrasonic bath and later into a vacuum filtration desiccator in order to expel the air introduced into the mixture during the mixing process and not affect the processability of the composite material obtained, giving stability to the material. compound.

. The composite material obtained, called 5%CCRNC<45, was spread in a photopolymerization tank to obtain 3D printed parts using the stereolithography technique. The pieces were washed with 2-propanol and air-dried for at least 24 hours.

. Subsequently, these parts can optionally be subjected to a curing process with UV radiation in a chamber for this purpose. This curing process Its purpose is to modify the mechanical properties of the pieces obtained with respect to the uncured pieces, observing different effects depending on the granulometry size of the cork powder. During the development of the example that illustrates the present invention, this optional process was carried out to verify the differences between the green and cured composite material, with significant differences in terms of Young's modulus, elongation at break and tensile strength. The conditions of the curing treatment were as follows: the fabricated samples were kept at the temperature of 60°C for 1 hour in a curing chamber. Figure 4 illustrates the data obtained for the example described, observing a notable difference in the mechanical properties of the printed composite material without curing treatment and with curing treatment.

The example that has been exposed has been carried out because it is the most representative according to the cited literature. In them, quantities of 5% (w/w) of cork are used with respect to the polymeric matrix as the most representative composite materials. It is necessary to highlight at this point that, although apparently an amount of 5% (w/w) by weight may seem like a low percentage, for the example shown, this represents an amount of approximately 25% by volume with respect to the resin. photocurable, taking into account for this calculation the density of the resin used 1.13 g/cm3 compared to the 0.24 g/cm3 of cork. This very low value for the density of the cork means that small percentages by weight represent a large percentage by volume of the cork powder with respect to the photocurable resin. In the present invention it is observed that percentages greater than 30% (w/w) have a negative effect on the processability of the composite material.

INDUSTRIAL APPLICATION

This procedure is scalable at an industrial level, and can be carried out in small and large format 3D photopolymerization printers.

Claims (6)

1. Material for use in stereolithography characterized in that it is composed of: a) a photocurable base resin for use in stereolithography, b) cork powder particles with a grain size of less than 150 ^m, where the percentage by weight of cork powder with respect to the photocurable resin matrix is less than or equal to 30% (w/w). 2. Material for use in stereolithography, according to claim 1, where the percentage by weight of cork powder with respect to the photocurable resin matrix is 5% (w/w) and the granulometry size of the cork powder is less than 45 ^m. 3. Procedure for obtaining a material for use in stereolithography, according to previous claims, characterized in that it has the following stages: a) Grinding of cork, of natural origin or from the cork transformation industry. b) Screening of cork dust, using a manual or mechanical sieve. c) Manual or mechanical mixing of the sieved cork powder with the photocurable resin. d) Introduction of the mixture obtained in the previous step in an ultrasonic bath and later in a vacuum filtration desiccator. e) Dispersion of the material obtained in a polymerization tank, obtaining 3D printed parts using the stereolithography technique. 4. Procedure for obtaining a material for use in stereolithography, according to claim 3, characterized in that an optional stage of curing with UV radiation is added, which produces a modification of the mechanical properties of the pieces obtained. 5. Procedure for obtaining a material for use in stereolithography, according to Claims 3 and 4, characterized in that using a percentage by weight of cork powder of 5% (w/w) and a granulometry size of the cork powder of less than 45 ^m, an increase in Young's modulus is achieved in the cured material relative to uncured material. 6. Procedure for obtaining a material for use in stereolithography, according to claims 3 and 4, characterized in that using a percentage by weight of cork powder of 5% (w/w) and a granulometry size of the cork powder of less than 45 ^m, a decrease in thermal conductivity is achieved with respect to the material without finely divided cork additivation.