The practice of flat architectural glass using as a building material is calculated for decades. During this time, manufacturers have made significant progress in the technology of manufacturing and processing. They have brought its key features and properties almost to perfection. Nanotechnologies clearly manifest themselves in glass technology, allowing to make specific, extraordinary performance and aesthetic qualities. They open up new prospects for architects and designers.
It should be noted that the most significant improvements of all glass properties and possibilities were able to open thanks to emergence of multi-layer safety glass products. All major valuable operational and technological properties of these glasses are achieved through the use of «sandwich» principle. It implies forming of inhomogeneous layered composite structure with an intermediate polymer layer. The meaning and implications of these developments are still difficult to assess.
Glass makes an attempt to be on a par with the basic construction materials such as steel and concrete.
Unfortunately, there are still many issues of technology and equipment improving to create a multi-layer glass (triplex equipment) remains outside researchers’ attention. This fact hinders seriously the practical development of whole range of safety glass possibilities. The majority of new features are hidden and it needs serious theoretical and experimental study. These include: a detailed study of formation mechanism, development of technology for producing a multilayer glass-based innovative polymeric materials and investigation of characteristics and application possibilities of hybrid structures based on new composite components.
The combination of fragile glass and plastic materials (polycarbonate-glass sandwich, acrylic glass block and many others) will optimize the functionality of laminated safety glass. There is no doubt that the development of research in these directions will encourage the further improvement of existing and development of new approaches in development of devices for industrial manufacture of laminated glass.
And so, without taking into account the differences in the scientific and technical interpretation of term «laminated glass», we will focus on the conventional definition:
Glass lamination — is a method of creating a stable combination of flat or bent glasses, bonded together over the entire surface, by liquid or polymeric film materials.
Currently, two technologies for production of all kinds of laminated glass are uses in the world: a technology of liquid pouring, based on usage of plasticized resin (filler), and technology when polymer film is used as a binder between layers.
Filler technology by virtue of technological and operational constraints takes narrow segment in the total production of safety glass.
In turn, film technology is divided into two groups: the autoclave and non-autoclave. Both techniques are based on the same physical principles.
Major differences in technical realization and sequence of process steps.
The manufacturer’s attention of sheet glass laminate has recently directed at non-autoclave film technology as economical factor plays mostly main role in our world. In addition, emergence of new films for non-autoclave laminating opens up the possibility of its use for architectural purposes, especially for hardened glasses.
Unfortunately, a serious drawback of both technologies is very high defect rate, which may be up to 10-15% of production output.
The main reason for poor quality of such products is usually the presence of «tack» and «bubbles» on the borders which appear due to local differences in values of adhesion strength on surface of glass sheets.
Therefore, the most important task in the production of laminated glass is problem of increasing uniformity and adhesion strength over entire contact surface of polymeric materials and glass. This problem can be called defining.
Certainly, adhesion properties depend on the chemical nature of polymeric materials included in multilayer glass composition. Moreover, from point of view of adhesive contact formation, maintaining of strict technological rules of preparatory processes is very important (washing and drying glass and films, the assembly and laying of package and degassing). However, as the theory and practice show the main reason for defects is originally incorporated in the heat cycle.
The reason for this is the specific thermal properties of glass, which result in uneven heating of multilayer composite. This is especially important when it is not a simple combination of glass-polymer-glass (so-called triplex), and the type of multilayer glass called polyplex. Theoretical and applied research in the field of glass heating reflected in a number of monographs and reviews. However, they focus on optimizing of technological operations at hardening, thermal hardening, bending of sheet glass.
It is generally recognized that creation of equipment for manufacture of safety glass is a complex technological problem.
In fact it can already be noted that some types and methods of glass heating in lamination ovens lose their positions, yielding a more progressive infrared heat.
Unfortunately, the complete lack of practical results and basic theoretical positions are currently a limiting factor in the development and implementation of this kind of heating in lamination ovens.
Thus, there are objective prerequisites for the following scientific and technological objectives solving:
In order to increase the efficiency of multilayer building glass production it’s necessary to develop a technique, a way to adapt and maintain a set of measures and operating conditions of lamination devices which will provide optimal conditions for formation of uniform, stable glass and intermediate polymer connection.
The problem of optimizing thermal operations in the production of laminated glass has repeatedly been considered previously, its solutions in various productions helped to develop a number of practical recommendations aimed at improving the characteristics of heat treatment process.
Summary of results of practical operation of lamination ovens of domestic and foreign performance shows that transfer from one type of glass to another or change the type of intermediate polymer film requires a serious adjustment of operating modes. Lamination operators must either implement optimization empirically, or to rely on technological modes which are recommended by films manufacturers (which doesn’t always have generalized nature).
In this case production flexibility is greatly reduced, and we can not exclude the possibility of defects. In addition, heat treatment modes selected on the basis of practical experience, are not the most efficient. As a consequence, all this has a negative impact on cost and consumer properties of products. Equipment developers need description of conditions and specifics of physical processes of formation of adhesive bonds.
You can draw an analogy — powder coatings, as well as films for lamination are also divided into two main classes: thermoset and thermoplastic.
With powder paints everything is clear: to form a coating you need thermal exposure (for thermoset), and recommended temperature of object heating (for thermoplastics). We have complicated combination glass-polymer-glass-film in case of glass lamination (for pressing we usually use silicone membrane with thickness of not less than 4 mm). Formally a task of lamination is reduced to bonding process: by heating of adhesive film and pressing.
The basic concept of selection and justification of rational modes of IR energy supply in lamination technology was studied and worked out on a specially designed combined IR installation using quick-response linear quartz infrared sources. General view of the installation is shown in the photo.
In the first stage of experimental device designing as trial experience, we tried to link the analytical problem of radiant heat transfer with the whole device, assuming a discrete location of IR emitters based on their design, dimensions and configuration of reflective screens.
The second important task — is to optimize dynamical regimes of complex glass and polymer sandwich heating with given boundary conditions and restrictions on the temperature gradient, taking into account spatial distribution of infrared sources temperature field. We identified the following contradictions arising from the optimization of dynamic modes of composite structure heating. On the one hand it is necessary that duration of heat treatment was minimal, and on the other hand temperature gradients must have a certain value. That is why the final value of each polymer layer temperature must match exactly the recommended polymer temperature.
Rapid glass heating creates excessive temperature gradients both in volume and in thickness of package that can cause uneven heating of polymer, and even to its boiling. Slow heating can substantially reduce gradients in package volume, but it significantly increases the time of heating to lamination temperature, which reduces device’s productivity.
In this regard, the task of dynamic mode of IR heating managing can be formulated as follows:
For given input parameters, you must find mode of infrared source at which each intermediate layer is heated to the melting point of the minimum time.
It has been hypothesized that one way of solving this problem is the use of pulsed infrared heating on the basis of flat glass and polymeric materials.
In connection with need to measure and control glass sandwich temperature, information-measuring system was developed. It manages pulsed infrared heating, which allows you to maintain the maximum and minimum temperature of the material.
Taking into account results of physical modeling, we have made industrial device for glass laminating, which is based on use of radiant heating and developed methods of volume leveling of temperature field.
(see. photo and specifications)
The device operates on principle of programmable infrared heating of vacuum-package «glass-film-glass»
Good results both in the quality of glass lamination and in energy consumption were achieved in operating process of lamination device GLD-4,1 / 2,5 IR. The possibility to laminate products of different thickness in oven with infrared radiation was experimentally proved. The adhesion, strength and optical characteristics are maintaining during this process.
In addition the results obtained while using this machine allowed to make certain adjustments to theoretical understanding of formation and management of polymer’s adhesion properties in the composition of glass and polymer sandwich. We have changed approaches to practical implementation of individual blocks and units in order to increase the effectiveness of this machine’s class. First of all it concerns optimization systems of oscillating mode of IR energy supply.
Summarizing it can be stated that the possibility and prospects of using pulsed shortwave infrared heating in lamination ovens was shown.
Continued work in this direction, apparently, will allow to develop correct practical recommendations, and thereby improve the quality of laminated safety glass.