Microwaves in natural stone resin treatment plants

Authors:
Flavio Sartor: Researcher at the Breton R&D Centre and Moreno Gnesotto:  Breton Sales Manager
 
Reasons for resin treatment
Numerous marbles and granites are characterised by natural internal fracture lines (fissures) and porosity.
These materials must be subjected to a consolidation process with reinforcing mesh on the rear face of the slab and resin enhancement treatment on the exposed face to seal any fissures, pits or micropores.

Specifically, mesh application is necessary only for extremely fragile materials with evident fracture lines and/or fissures.
In contrast, resin treatment of the exposed face (also called enhancement), has become a widespread practice used for almost all stone materials because it brings out the aesthetic qualities and colours of the stone, reduces porosity and increases stain resistance, simultaneously consolidating any internal fractures in the stone.
These processes are performed using structural resins, primarily epoxy systems and secondarily unsaturated polyester resins.

The resin treatment process
An important precondition for properly executed resin treatment is for the slab to be perfectly dry so that the resin can anchor securely to the dry stone surfaces.
When choosing a resin plant special attention should be paid to the drying section (which is normally achieved with recirculated dry hot air) and to the method of drying (parameters to consider include slab holding time in the drying station, air temperature, air velocity, air/stone incidence angle, and so forth). 

The research staff at the Breton R&D Centre have tested a large number of different resin formulations and analysed all the associated process parameters.
Specifically, the analysis concerned several different resin treatment systems (atmospheric, vacuum and dip) and various resin types, namely epoxies, acrylics and polyesters, suitably modified in accordance with the required characteristics: fluidity and degree of penetration, adhesive strength, colour fastness, durability of properties, resistance to weathering, UV resistance, etc. We have also analysed various resin curing systems: infra-red, hot air, ultraviolet, radio frequency and, more recently, microwaves.
Attention was focused on the pros and cons of the various techniques in terms of efficiency, results, and process costs.

Slab drying

The slab drying process is a crucial step because organic resins are not compatible with water. Any water retained in the stone will prevent direct contact between the resin and the inner walls of fissures, impairing adhesion and compromising the consolidation process and its efficacy through time.
We know that that higher air circulation velocities and air temperatures result in faster drying of the stone. However, experience also shows that excessively high temperatures can produce stresses and failures in machined stone slabs. 
That's why we studied the various slab drying techniques (static drying with forced hot air circulation and dehumidification, feed through drying with infra-red radiation, static drying with radio frequency radiation, etc.), identifying the relative pros and cons in terms of both efficiency and process costs. The most suitable drying techniques should be selected in accordance with the material to be treated.
For example, the following graph shows the result for drying a wet granite slab in a static oven with forced hot air recirculation at 50°C, which shows that after 80 minutes the slab can be considered to have reached an optimal level of dryness to proceed with resin treatment. 

Mesh application on the unfinished surface of the slab  

Once the slab is dry we can proceed with mesh application, if required: this process involves the application of a fibreglass mesh on the rear of the slab using epoxy resin, which gives optimal adhesion. This operation is required when the fissures in the slab are such as to impair subsequent handling and work processes on the material.

Resin treatment of the exposed face of the slab
This operation must be carried out on a polished and clean surface. Fine grit honing of the slab makes it possible to minimise the quantity of resin required and reduces the thickness of material to remove thereby avoiding fresh areas of porosity being uncovered. Washing is required for some types of stone to clean out micropores and fissures and prepare them for resin penetration and adhesion.

Resins
Epoxy resins are generally made from two components: the resin (part A) obtained by the reaction between bisphenol A and epichlorohydrin, and the hardener (part B), which is normally a mixture of amines. The hardening reaction occurs due to polyaddition between the molecules of the two components to form the final product. Epoxy resins feature high adhesive power, excellent wettability, low shrinkage during hardening, good transparency and a moderate level of resistance to weathering. Transparency and colourless appearance depend on the type of hardener used. Specifically, hardeners that produce colourless resins with enhanced weathering properties are more expensive and the hardening process is slower. In general epoxy resins are more expensive than polyester resins.
The polyester resins currently used are generally esterified copolymers containing unsaturated chains dissolved in a vinyl monomer (usually styrene). When the resin is combined with the activator, resulting in opening of the double bonds, the consequent cross-linking reaction causes it to harden. These resins are characterised by their long working time and, depending on the type of activator, rapid hardening at the predefined temperature. Polyester resins also feature lower cost, easy handling, good transparency and good resistance to weathering.
The choice of the type of resin to use depends mainly on the type of stone to be consolidated, although there is currently a near universal preference for epoxy resins, which ensure enhanced resin/stone adhesion.

Resin hardening method
Both types of resin will harden without requiring external energy once they have been activated. However, current plant design requirements call for the fastest possible hardening time without impairing the results of the consolidation process. So to speed up the hardening reaction we need to supply a certain quantity of energy. Energy can be supplied in various ways: irradiation (infra-red. ultraviolet, microwaves, etc.) or by convection/contact heating (hot air).
The most widely employed systems make use of vertical static ovens with forced hot air ventilation to cure epoxy resins, and infra-red or UV ovens to cure polyester resins.
Depending on the type of resin used, the material to be treated (marble or granite) and the production capacity required, the most suitable curing system must be studied and proposed.
For all resin types, hardening must be followed by a subsequent ambient temperature maturing stage of varying duration to allow complete catalysis/hardening of the resin before proceeding with slab polishing processes.

CURING OF EPOXY RESINS WITH MICROWAVE SYSTEM

For resin treatment with epoxy systems curing is currently carried out in static multiple tier ovens with forced air circulation (approximately 50°C), with slab holding time in the oven of around 80 minutes. In general, following oven curing the customary 24 hour period is allowed to elapse before proceeding with the polishing stage.
Recently, technological evolution has made it possible to positively consider also the use of curing microwave systems.

That's why the Breton Research Centre has been performing detailed studies to assess the capabilities and efficacy of using microwaves to accelerate resin hardening.
It should be emphasised that microwave curing ovens are rather different from normal ovens in that the former are complex, carefully studied and calibrated systems that combine microwave radiation with hot air convection heating.
During the research project we used various types of commercial resins: normal epoxy systems and "specific" epoxy systems designed for microwave curing.
The test results showed that microwave curing epoxy resins are highly reactive and display greater receptivity to microwave radiation. It was found that the combined microwave/hot air system effectively accelerates heating and successive hardening of the resin.
The annexed graph shows the exothermic reaction curve during hardening of two epoxy resins at 50°C, one of which is a normal resin and the other a microwave curing resin.

It should be noted that microwave curing epoxy resins are mainly resins with slightly yellower colouring than other types of resins utilised in the normal resin treatment of granite slabs, so due consideration must be awarded to this characteristic.
The necessary condition to ensure rapid and complete curing with microwaves is to reach a resin temperature above 60/70°C. This clearly causes also the temperature of the stone to increase. Also this factor requires careful consideration, since it is known that heating the stone can be deleterious in certain cases due to the consequent effects of thermal dilation and "widening" of fissures in the stone.
It's also important to consider the shorter pot-life (the interval from addition of hardener to the resin to the time when the resin starts to harden) of microwave curing epoxy resins, since this affects the viscosity and the time available for the resin to penetrate into fissures and pores.

Hardening tests of resins with different heating systems, showed that materials treated with microwave curing epoxy resins can be polished after a shorter interval when the resin is cured with microwaves combined with hot air.
The following graph shows the surface hardness of different resin types after hardening (measured with a Shore D scale hardness tester). It is known that surface hardness is a key parameter in relation to the success of the polishing process.

Epoxy resin curing with microwave systems: final conclusions
Use of the microwave system (note that microwave curing ovens are different from normal ovens in that the former are complex, carefully studied and calibrated systems that combine microwave radiation with hot air convection heating) for curing resins is for sure an innovative and valid technique that is suitable both for mesh application on the rear face of the slab and for resin treatment of the exposed face (so called "enhancement") for materials that are not sensitive to overheating or to the use of a slightly yellower resin, and when production requirements call for rapid polishing without having to wait the usual 24 hours associated with epoxy resins cured in the conventional manner using exclusively hot air ovens.
Regardless of the treatment, type of resin and curing method, in order to obtain optimal resin penetration it is always necessary to guarantee complete drying of the walls of fissures. As shown in the relative graph, the drying stage is extremely delicate and absolutely essential for the success of the entire process; in physical terms it calls for a well-defined hot air temperature/oven holding time curve in order to achieve adequate drying of the stone.
The Breton Sales Department and Research Centre are at the disposal of stone processors seeking the best resin treatment system for their needs and materials.

Breton has sold to a primary company in the province of Verona, CERESER MARMI, the first industrial microwave technology mesh application and resin treatment plant, scheduled to come on stream this coming October.
It's hardly surprising that Breton's motto is:
“Driven by innovation”

Flavio Sartor & Gnesotto Moreno

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