Stereolithography and Photo Polymerisation - Essay Example

12 September Stereolithography and photo polymerization Stereolithography is among the many methods used in object building additively in 3D printing technology. Stereolithography is mainly used for the process involving rapid prototyping and manufacturing (Jacobs). The use of the technology is focused in the translation of computer Aided Designs to its real solid equivalent. This is facilitated by a combination of software technologies, combination of laser and photochemistry. Stereolithography is preferred my many engineers and manufacturers due to the various advantages associated with its use (Bártolo). Some of the advantages associated with Stereolithography include its low costs, production of durable objects, efficiency and its high precision (Bártolo). Basically, the process of building an object using the Stereolithography entails the creation of a 3 – D model of the object using a desired CAD software, using a software such as lightyear in slicing the produced 3 – D model into series of horizontal slices (Thin slices). An ultraviolet sensor then scans the photosensitive resin’s top layer thus hardening it (Miles, Cillo and Sinn). This builds a new layer which is then attached and lowered below the surface covering the distance of one layer. A new layer is then coated on top of the previously scanned layer and the process repeated till the final product is produced. Figure 1: Components of Stereolithography process (Thre3D) In the process of printing, a laser is used in drawing the models layers, each at a time, to a photopolymer resins and in the process, each layer is cured at a time. In this process, there is projection of light beam – UV light – in form of a laser on the resin at a specific point and as a result, the parts of the resins reacts and then solidifies. This is then followed by the laser drawing the object’s cross-section resulting in a hardened layer (Newnes). The final products of the Stereolithography process has various properties which include water resistance, high impact resistance, and thermal resistance among other desired properties. As seen in the above description of the process of the construction of an object using Stereolithography method, it can be seen that light plays an important role in the solidification of the photosensitive resins. The above description is based on a construction using the laser lithography (Stereolithography) but this can be achieved, also, using other technologies such as the Photo – Mask technology. The creation of objects using these technologies is based on the principle of Photopolymerisation. This section of the paper is dedicated to provide in - depth research on the principal of operation of Photopolymerisation. Photo – induced polymerization, otherwise known as Photopolymerisation involves the absorption of Ultra – violet light resulting into the production of unstable and excited species. Subsequently, polymerisation results due to free radical formation through rearrangement, fragmentation and attack by a monomer (Fairbanks, Schwartz and Bowman). Basically, the initiation of the process of polymerization is by light and the end of the chain is signalled by the formation of free radicals. In this process, radicals are generated by initiators referred to as photo – initiators (this are compounds added to a formulation specifically to facilitate the conversion of the light energy induced by the UV light to chemical energy in a form of free radicals). The absorbed UV light is responsible for the breakage of the carbon bonds on the monomer resulting into formation of radicals (two). The radicals react with a monomer initiating radical polymerization (Korkin and Rosei). The illustration below depicts this process. Figure: Initiation of radical polymerization (Korkin and Rosei) There are two classes of photo – initiators basing on the type of the mechanism used in creating the radicals. This are the Type I photo - initiators and the type II photo – initiators (Lewis). Type I photo – initiators are also known as unimolecular photo – initiators. Substances belonging to this category when in use, upon absorption of light, undergo homolytic bond cleavage. The most commonly compounds used in this category are Benzoin and its derivatives (ScholarlyEditions). They are mostly used in the polymerization of vinyl monomers. The image below represents the polymerization of benzoin. Figure 2: Type I Photo initiators (Yagci) The use of the type II photo – initiators also referred to as bimolecular photo – initiators involves the use of photo – initiating systems that have certain photo – initiators such as benzophenone and a co - initiator like an amine or an alcohol. The generation of radicals is by a bimolecular process which entails the reduction of photo – excited aromatic carbonyl compound. The reduction is by electron transfer reactions or by the process of hydrogen abstraction. The figure below depicts the process of Photo initiation using type II photo – initiators. Type II photo initiators are best suited for 2-carboxy ethyl acrylate Figure 3: type II photo initiators (Yagci). Another form of photo - polymerization is the photo – initiated cationic polymerization. There are two types photo – initiated cationic polymerization systems. These are the direct acting systems and the indirect acting systems. In the direct acting systems, photolysis of these initiators – which are stable thermally and hygroscopically – goes through an irreversible photo - fragmentation resulting into production of Bronsted acids and Cation radicals. The cationic polymerization containing suitable monomers is initiated by the reactive species resulting from the photochemical process and combined with onium salts. The illustration of this process is shown in figure 4 below. Figure 4: Photo – initiated cationic polymerization direct acting systems. In this direct acting systems, cationic polymerization is initiated by the salts upon irradiation. Despite this, the simple onium salt spectral response is to UV light (specifically the shorter region). Conversely, the spectral sensitivity is extended to longer wavelengths in the indirect Acting systems. These systems are used to remove the limitations associated with the use of the direct systems. The figure below illustrates light absorption in this systems. Figure 5: Indirect Acting system extended spectrum (Yagci) The expansion of the wavelength wave is achieved in various ways. These methods include; a) Use of either photo – excited sensitizers or free radicals in electron transfer reactions, b) Using electron donor compounds. Indirect initiating systems have different classes of modes of action, specifically 3, in the formation of cationic specifies that have capabilities of reacting with monomers. These include the oxidation of free radicals (Lewis). In this mode, the onium salts oxidizes the photolytically formed radicals (Jacobs). In the process, cations are generated and these cations are utilized as initiating agents in the process of cationic polymerization. This is illustrated in the equation below. The second mode is the transfer of electrons between an onium salt and photo – excited molecule. This process involves sensitization of aromatic hydrocarbons to decomposition of onium salts. This is achieved through an excited complex electron transfer, exciplex. The process of exciplex involve a single electron being moved from to the onium salt from the sensitizer molecule and as a result, sensitizer radical cations are generated (Lewis). The whole process can be illustrated in the equation below. The third mode involves charge-transfer complexes. In this mode certain onium salts undergo electron-transfer forming aromatic radical cations as illustrated below. The action of the process of Stereolithography uses the same principle as the process of two – photon polymerization. The action of these two process is based on the principal that presence of light has capabilities of triggering chemical reaction and consequently resulting in polymerization of the photosensitive material in question. Two – photon polymerisation involves the process by which three dimensional structures are fabricated using resolutions of approx. 100nm (Änderung). In the process, resins undergo reactions by absorption of two photons simultaneously. Two – photon induced photo – polymerisation is a vital process that has capabilities of polymerizing structures even those with sub – micron features. This gives the technology diverse application in the process of 3D printing. This technology has emerged as the best technology among the various 3 Dimensional micro - fabrication methods. Two – photon polymerization together with ultra-short laser pulses when focused onto photo sensitive materials, there is initiation of polymerization by the light pulses by two – photon absorption and consequently, resulting in 3D polymerization (Fairbanks, Schwartz and Bowman). After the desired structure has been illuminated and the curing process done, the materials (Polymerized material) structure remains in 3D form. Therefore using the technology, it is possible to generate any 3 - D structure from computer generated 3 – D models by the use of the direct laser (Korkin and Rosei). The process is characterized by non – linear nature and threshold behaviour. Therefore, to attain a resolution that goes beyond the diffraction limit, the laser pulses energy can be controlled together with the number of applied pulses. These consequently makes the use of the two – photon polymerization technology to provide high quality structural resolution as compared to other technologies like the Stereolithography (Änderung). The application of the two – photon polymerization technology in the creation of 3 – D objects has been found to be extremely effective when 3 – D structures that have a resolution of more than or equal to 100 nm are fabricated. In the technology during material processing, there is the utilization of computer controlled systems in conjunction with light sources – typically infrared red light source (Bártolo). The accuracy and high resolution inherent properties of the process of two – photon polymerization can only be achieved by use of positions systems that are very accurate (these include scanners and piezometric stages). References Änderung, Letzte. Two photon polymerisation (2PP). 16 March 2012. Online. 16 July 2014. Bártolo, Paulo Jorge. Stereolithography: Materials, Processes and Applications. London: Springer Science & Business Media, 2011. Document. Fairbanks, Benjamin D., et al. "Photoinitiated polymerization of PEG-diacrylate with lithium phenyl-2,4,6-trimethylbenzoylphosphinate: polymerization rate and cytocompatibility." Biomaterials (2009): 6702–6707. Print. Jacobs, Paul Francis. Stereolithography and other RP&M technologies: from rapid prototyping to rapid tooling. Dearborn: Society of Manufacturing Engineers, 1992. Document. Korkin, Anatoli and Federico Rosei. Nanoelectronics and Photonics: From Atoms to Materials, Devices and Architectures. New York: Springer , 2008. Document. Lewis, Sandralee Patricia. Polymerizable and Polymeric Type I and Type II Photoinitiators. City University, 1992. print. Miles, Brett A., Joseph E. Cillo and Douglas P. Sinn. Stereolithography in Treatment Planning for Craniomaxillary Surgery. texas: University of Texas Southwestern Medical Center at Dallas, 2005. Document. Newnes. Comprehensive Materials Processing. Newnes, 2014. Document. ScholarlyEditions. Organophosphorus Compounds—Advances in Research and Application. ScholarlyEditions, 2013. Document. Thre3D. HOW STEREOLITHOGRAPHY (SLA) WORKS. 2014. Online. 15 July 2014. Yagci, Yusuf. Photopolymerization. Istanbul, 2010. Document. Read More