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6ИННОВАЦИОННЫЕ РЕШЕНИЯ
INNOVATIVE SOLUTIONS
The article has entered in publishing office 08.04.10. Ed. reg. No. 773 Статья поступила в редакцию 08.04.10. Ред. рег. № 773
PHOTOPOLYMERS FOR INDUSTRIAL HOLOGRAPHIC APPLICATIONS
R. Hagen1, T. Rolle1, T. Facke1, V. Ristic2
'Bayer MaterialScience AG, D-51368 Leverkusen, Germany e-mail: rainer.hagen@bayerbms.com 2Bayer MaterialScience, 3rd Rybinskaya Street 18, Bld. 2, 107113 Moscow, Russia e-mail: vladimir.ristic.vr@bayer-ag.de
Referred: 10.04.10 Expertise: 12.04.10 Accepted: 15.04.10
The demand for functional films and functional coatings which offer additional benefits to their users or which can combine several tailored properties and functions is growing. In converging markets like the security and IT markets, they help to reduce complexity and therefore cost of semi finished goods and end products. In the area of photonics, new optical and photo-optical films will allow new technical solutions in the near future.
A new class of dry photopolymers was developed by Bayer MaterialScience which shows high optical quality, excellent holographic properties and ease of use. A mechanistic model of these materials is discussed and the variety of today's and coming applications is sketched.
Keywords: photopolymer, volume holography, optical variable device, 3-dimensional image, holographic-optical device, HOE.
ФОТОПОЛИМЕРЫ ДЛЯ ИСПОЛЬЗОВАНИЯ В ПРОМЫШЛЕННОЙ ГОЛОГРАФИИ
Р. Хаген, Т. Роэлле, Т. Фэке, В. Ристич
Заключение совета рецензентов: 10.04.10 Заключение совета экспертов: 12.04.10 Принято к публикации: 15.04.10
Спрос на функциональные пленки и функциональные покрытия, которые предоставляют пользователям дополнительные возможности или которые объединяют в себе ряд заданных свойств или функций, сегодня растет. В специализированных областях, например, в области безопасности или информационных технологий, они позволяют упростить конструкцию и, следовательно, уменьшить стоимость полуфабрикатов и конечной продукции. В области фотоэлектроники новые оптические и фотооптические пленки в ближайшем будущем обеспечат развитие новых технических решений.
Подразделением Bayer MaterialScience был разработан новый класс простых в использовании сухих фотополимеров, обладающих высоким оптическим качеством и превосходными голографическими свойствами. В статье рассмотрена механистическая модель этих материалов и обрисован спектр существующих и перспективных направлений их использования.
Ключевые слова: фотополимер, трехмерная голография, оптически вариативное устройство, трехмерное изображение, голографическое оптическое устройство, голографический оптический элемент (ГОЭ).
Organization: Bayer MaterialScience AG, Leverkusen, Germany, Dr. rer. nat., Member of the "Deutsche Physikalische Gesellschaft".
Education: Physics Classes at the University of Bayreuth (1989-1993), Dipl. Phys. (1995), Dissertation in Experimental Physics (1998).
Experience: Research Associate of Prof. Dr. Dietrich Haarer, University of Bayreuth; Bayer AG, Scientist at the Central Research Dept. (1998-2003); Bayer MaterialScience AG, Innovation Manager and Trend Scout at the New Business Dept., Project Manager and New Applications for Photopolymers (since 2005).
Main range of scientific interest: experimental physics (laser optics, electro-optics, volume holography), functional polymers.
Publications: 20 scientific publications, >34 patents and patent applications.
Rainer Hagen
Thomas Rölle
Thomas Fäcke
Organization: Bayer MaterialScience AG, Leverkusen, Germany, Dr. rer. nat., Feodor-Lynen-Fellow of the Alexander von Humboldt Foundation.
Education: Chemistry classes in Marburg and Bologna (1989-1993), Dipl. Chem. (1994), Dissertation in Organic Chemistry (1997).
Experience: Research Associate of Robert H. Grubbs, California Institute of Technology, Pasadena (1998-1999); Bayer AG, Central Research Dept. (1999-2003), Bayer Healthcare AG, Medicinal Chemistry (2004-2005); Bayer MaterialScience AG, Innovation Management, Material Development for Photopolymers (since 2005).
Main range of scientific interest: organic chemistry, photochemistry, radical polymerization, polyurethanes, homogeneous catalysis esp. olefin metathesis.
Publications: 19 scientific publications, >35 patents and patent applications.
Organization: Bayer MaterialScience AG, Leverkusen, Germany, Dr. rer. nat.
Education: Chemistry classes at Philipps University of Marburg (1988-1992), Dipl. Chem. (1993), Dissertation in Nuclear Magnetic Resonance Spectroscopy (1995).
Experience: Bayer AG, Central Research Analytical Dept. (1995-2000), Bayer AG, Business Unit CAS, R&D UV Curable Resins (2000-2002), Bayer Corp., Technical Sales Powder Coatings (20022006), Bayer MaterialScience AG, Technical Sales PU-Dispersions (2006-2007), Material Development for Photopolymers (since 2007).
Main range of scientific interest: photochemistry, radical polymerization, isocyanates, coatings technology.
Publications: 25 scientific publications, >35 patents and patent applications.
Vladimir Ristic studied Industrial Engineering and received his B.Sc in mechanical engineering in 1996 at the University of Belgrade (Serbia). He also holds a master's in business administration from the Faculty of Economics, Finance and Administration (FEFA) in Belgrade. After studies he started his industrial career in rubber industry, company FGP Rekord in design equipment bureau and then worked as technical marketing manager in company UTI. He joined Bayer Serbia in 2003 as sales manager and was later responsible for marketing and sales in Southeast Europe. Then he worked at Bayer MaterialScience Germany in Business Unit "Coatings, Adhesives and Specialties" in marketing and sales. Currently, he is the country representative of Business Unit "Coatings, Adhesives and Specialties" in CIS responsible for marketing and sales of products for coatings, adhesives, functional films and carbon nano tubes.
Vladimir Ristic
Photo-optical polymers and supporting mega trends
The challenge for the polymer manufacturers and for the coatings and polymer processing industry in general is to translate market needs into technical solutions. For Bayer MaterialScience the starting point for a big success story in new optical applications were the Compact Disc (CD) and the Digital Versatile Disc (DVD) which started in 1982 and 1996, respectively. Digital data recording and retrieval became a mega trend. Nowadays the concepts for the already fourth generation of optical discs are developed. Holography is considered as one of a few technical routes with the potential for high data capacity and excellent archival lifetime. Bayer MaterialScience intends to participate again with its polymer grades, now with new photo-optical polymers for holographic data storage. The material development will be the main driving force for the realization of future disc media.
In another industry area, namely the industrial and residential lighting, new LED (light-emitting diodes) and O-LED (organic LED) light sources will help to improve
spectral light quality and will help to reduce energy consumption dramatically. Again, new photo-optical polymers can be implemented into newly designed light sources for a smart and efficient light-guiding and light-mixing.
On a higher technical level the related photopolymers and the same functions can be utilized to improve the efficiency of flat-panel displays. Furthermore, films and coatings of such polymers can pave the way for technically less complex active-matrix displays, and thus help to reduce installation space of displays and in a consequence reduce the total manufacturing and integration cost. To summarize: They will support the mega trends for reduced complexity, miniaturization and energy saving.
The fourth mega trend we consider as being relevant is headlined "Security". The market segments are: Brand protection, product tracking, document security and secure identity checks. Photo-optical polymers can form optical variable devices (OVDs) by utilizing holograms. Such OVDs can be integrated for example in tags and
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labels for efficient brand protection, into ID cards for a secure identity check of humans, and can protect banknotes and security paper against forgery.
In an outlook the raw material manufacturers will have to be prepared for a new customer demand: Future customers will increasingly require photo-optical polymers with, compared to conventional polymers, a broader spectrum of properties and functionalities. Important new properties are light-guiding, light-changing, and information storing. A very old and in this regard powerful technology is the volume holography: It guarantees a high selectivity in light color and in light propagation direction and therefore perfectly fits to the technical demand and the technology roadmaps outlined above. The resulting guideline for the material manufacturers is to provide high-end holographic films which are suitable to industrial roll-to-roll manufacturing and which can be recorded with common industrial lasers throughout the visible spectrum. The corresponding full color sensitized photopolymers are described in the following section.
Chemistry & technology of photopolymers
The development of new photopolymers within the Bayer MaterialScience "Coatings, Adhesives & Specialties" organization is an example for an innovative approach and is founded on an extended application perspective. We start with a definition of photopolymers and explain afterwards the chemistry and working principals of BMS photopolymers.
The term "photopolymer" is used for all kinds of polymers which are processed by actinic light or generally by electromagnetic radiation, and accordingly polymers which show light-induced chemical processes, so-called photochemical reactions. From their fields of use they can be classified into three different groups with different markets and industrial relevance:
The largest group by sold material consists of UV-and electron-beam curable coatings and adhesives, e.g. for wood applications, paper and plastic coatings. The world-wide sales volume of such photopolymers exceeded 250,000 tons in the year 2007.
Other photopolymers, the photoresists, are made for photo- and e-beam lithography [1]. The challenge for the material developers in this sophisticated technical area is to allow higher and higher lateral resolutions in order to meet the demands of the semiconductor industry for smallest integrated circuits. As a consequence of the high resolution, the photopolymers are processed with high-energetic radiation of up to 90 eV. The tradeoff between energy dose, line resolution and precision of structures has to be balanced. Chemically the photopolymers for lithography are often cationic polymerizing compounds. A lot of innovation deals with the nature and design of the photoinitiators: Many photoresist manufacturers deal with hypervalent iodonium and sulfonium compounds with high quantum yield.
The third group of photopolymerizable materials is the one of interest for this paper: Photopolymers for holography. They have been intensively investigated for many years, as they are excellent candidates for Write-Once-Read-Many (WORM) volume holographic storage devices. It is their ability to be tuned to specific lasers and recording scenarios. The photosensitivity and the dynamic range combined with a high dimensional stability are the measures by which these materials can be properly judged. Excellent performances in these parameters make photopolymers very interesting candidates for recording materials in future high-capacity optical discs [2-4]. Outside of holographic data storage, photopolymers are highly suitable for visual color holography and for holographic-optical elements (HOEs). A variety of applications is thinkable from advertising and flat-panel displays to document security and brand protection, and a few of them have already been realized.
For many years, holographic scientists have utilized photopolymer materials [5], silver halide emulsions [6] and dichromated gelatines [7]. All have produced shiny color holograms that still can excite the public as well as the professional users. The latter two emulsion types are very difficult to process and require lots of tedious handling steps. Despite all efforts to optimize photopolymer products for hologram mass production, they still require a lengthy thermal development after the holographic recording to develop and fix the holograms. Dependent on the hologram production volume and the hologram replication and conversion process, this thermal step can reduce the hologram output or process reliability.
Bayer MaterialScience develops a class of full color photopolymer materials. Their main product properties are: RGB capability, easy photonic recording and fixing, mechanical robustness, high temperature stability and durability in the application. However, the unique benefit against current photopolymers is their pure photonic processing with laser light and UV/VIS curing light without the need for baking or other processing.
Matrix
ТУ Photoinitiator
о Writing Monomer
Stages of holographic grating formation in a photopolymer film, recorded in a two-beam interference setup: 1 - hologram exposure (yellow = intensity grating); 2 - radical formation and photopolymerization; 3 - chain growth and monomer diffusion;
4 - hologram formation (background color depicts refractive index modulation; dark=high)
З
2
4
7S
The basic working principle of Bayer's photopolymers is shown in Figure: binders, monomers, photoinitiators and sensitizers (the "writing chemicals") are dispersed into a polymer matrix. When exposed to light (1), the photoinitiator enables the formation of free radicals (2). These free radicals then interact with neighboring monomer molecules. The monomers photopolymerize during recording (3). In a standard fashion the holographic grating is a regular intensity pattern, generated by two-laser beam interference. The index modulation follows the light pattern (4). Afterwards the holographic grating is permanent and can be viewed with the eye or machine-read.
The performance indicators of any volume holographic photopolymer are high quantum yield during photo-recording, efficient photopolymerization, high refractive index modulation, and high optical transparency and quality of the as-prepared film. In order to reach a high level at all of the above mentioned performance indicators, Bayer developed and selected appropriate raw chemicals and optimized the processes of photochemical development.
The Bayer resin consists of matrix pre-cursor and imaging components. The resin can be coated, e.g. on a polycarbonate (PC) or polyethylene terephthalate (PET) substrate. During recording the cross-linked matrix self-develops. Back again to Figure: Macro monomers will grow in the bright regions of the holographic fringe. They consume locally an increasing number of monomers. This leads to a diffusion of monomers from the dark into the bright regions. As the matrix network has an index of refraction different from the orthogonal polymerizing monomers, the mass transport or molecular drift supports the formation of an index modulation in the material which follows the light pattern. Light scatter through inhomogeneities is successfully suppressed. The material development is also considering that polymerization chains should not migrate too far into the dark regions of the fringe.
The strength of this unique two-chemistry approach is that it allows for an independent optimization of matrix and imaging components. Bayer MaterialScience has developed the photopolymer material and respective film products. It expects to provide the first industrial grade in summer 2010 and is heading for a breakthrough in many industrial applications in the mid- to long-term.
The relevance of holographic photopolymers for the industry
Photopolymer materials have recently received a great deal of attention in areas such as holographic data storage, 3-D displays, and integrated optics. The fact that volume holography was invented more than 60 years ago by Dennis Gabor et al. helps, as a lot of know-how about holographic processing and optimizing is available in research labs. This is especially true for Russia, where
clustered expertise is available because in-depth know-how was developed and maintained over years since the Russian physicist Juri Nikolajewitsch Denisjuk invented the white light holography in 1965 and made holograms very attractive.
Despite its long history, volume holography could not bring a commercial success except some brand security label and ID card applications. The industry mainly relies on embossed, rainbow-type holograms and developed in the 1980's mastering and mass replication technologies. The total annual sales with holograms are at about 2.5 billion US-dollars today [8].
New applications like autostereoscopic 3-D displays [9], head-mounted displays [10], optical waveguide couplers [11], light-guiding elements in photovoltaic cells [12] and reflectors for LCD panels [13] are expected to reach the commercial stage within this decade. Additionally, volume holograms could be part of next generation 3-D advertising posters, projection screens, ID cards and security labels. One indicator is the growing number of patents in these fields. Other indicators are the congruent roadmaps for micro beamers, LED power light and cost-efficient diode-pumped solid-state lasers, and integrated optical devices like modulators and demultiplexers. These key technologies will be available in time to drive the new holographic applications.
Experts consider the availability of durable and high-performing photopolymers as main requirement for any commercial success. Bayer MaterialScience plans to offer durable self-developing holographic film products. The long-lived dream of holographers to have access to easy processible high-quality recording films can come true. Bayer MaterialScience can thus support a sustainable growth in new photonic applications which will be beneficial also for the non-optical industries worldwide, too.
Conclusion
Based on its experience in optical data storage, Bayer MaterialScience has started in the year 2006 a development of holographic photopolymers. The result from this development is a new class of photopolymers as holographic recording media which will be made available to Business-to-Business customers like high security printers.
The Bayer photopolymer products are suitable for several applications and markets like brand protection, document security, consumer electronics, and display holography. The utilized holographic functionalities will be different, reaching from optical variable devices for security applications to holographic-optical elements for screen and display applications. Such holographic elements will thus contribute to higher product security and quality, and enable completely new applications in the future like 3-D television or semi-transparent projection screens.
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Bayer MaterialScience continues to work with customers to further support these development efforts, to tune and optimize its photopolymer films needed to achieve the required performance in the specific application and to adapt to the manufacturing processes.
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