27.2 Chemistry and research- Adhesion and flexibility: Epo 155

29/07/11

When in 2005 the epoxy resin Epo 155 was included in CTS catalog, it was believed that its use would have been limited to two cases: bonding of materials subjected to strong dimensional variations (related to changes in relative humidity, as for wood), and joining of materials characterized by very different thermal expansion coefficients. In fact, in these cases, the use of a traditional epoxy resin, that is rigid, might create stress tensions at the interface that may damage bonded feature when that is too weak. The well known example is the bonding of a metal element on a degraded stone, which in the long run may have micro-cracks or delamination of the bonding area. In fact, in these years Epo 155 showed to be suitable for other applications. Here are a couple of interesting cases: a comparative study on Epo as a consolidation agent for wood, and one more example of an unexpected application. We d like to underline that, among the cold cross linking epoxy resins, Epo 155 is a two component product, and needs to be mixed with a hardener based on modified aliphatic amines.
The properties are summarized in the table below.

Appearance of the mixture EPO + 155 K 156                 Transparent liquid 
Specific weight at 25 °C (Kg /l )                                              1.1
Working time at 25 ° C                                                           30 '
Tensile strength (N / mm2)                                                   > 3.0
Flexural strength (N / mm2)                                                  > 3.5
Compressive strength (N / mm2)                                           > 5.0
Viscosity of the system at 25 ° C (mPa.s)                           120-240
Modulus of elasticity (N / mm2)                                             1800


As you can see from the given viscosity data, the resin is very fluid, and this led to some recent studies by Enriques 
et al [1],[2] focused on the consolidation of wooden architectural elements attacked by mushrooms. In addition to two classic resins such as Paraloid B-72 (acrylic resin) and the Butvar B-98 (polyvinyl butyrale), four more epoxy resins with low viscosity were tested, including, of course, Epo 155, and applied on samples of pine degraded by wood-decay fungus. Different periods of time under the action of the mushrooms (4, 8 and 12 weeks) resulted in different levels of decay connected to the loss of mass (between 3 and 25%). The application was carried out by direct immersion in the epoxy resins, for 15 ', while the two thermoplastic resins were dissolved in acetone (20%) and in a mixture of ethanol/toluene 40:60 (15%), respectively.
More details are reported in the extended article, anyway it is interesting to extrapolate the table that shows the increase of the mechanical resistance to compression, in comparison to that of the natural samples (for simplicity only the values obtained at a low and to a high level of degradation are reported, that means 5% and 20% of mass loss).


Mass loss             Epoxy             Epo 155             Epoxy             Epoxy             Paraloid             Butvar
                               R                                            Lw                   Li                  B-72                  B-98
       5                     7.0                    17.2                10.7                 8.2                  12.7                   15.1
      20                   -3.8                    38.5                 29.6               15.4                  26.6                   36.6

As you can say, epoxy resins give negative results, most likely because the poor penetration in the wood matrix; the best results are obtained with Epo155 and Butvar. Anyway, Butvar is not able to improve the
 resistance of the surface as demonstrated by the next table.


Mass loss               Epoxy             Epo 155             Epoxy             Epoxy             Paraloid              Butvar
                                 R                                             Lw                    Li                 B-72                  B-98
       5                       7.6                   32.9                  12.5                14.4                14.9                   6.8
      20                      19.8                 90.8                   22.3                13.2                26.9                   2.5

The authors point out that only in some cases there is a relationship between the amount of absorbed resin and the resistances: for example Paraloid B-72 is absorbed to a limited extent, but there is a growth of 27% for the compression and for the surface as well. The absorbed quantity of Butvar is even less; that gives a good result only for the resistance to compression. The epoxy called R is the resin that is absorbed most, but its effectiveness is poor.In conclusion, Epo 155 is a product that gives the best results, considering all the parameters. It is therefore suggested as an alternative to Paraloid B-72, especially when reversibility is not needed.

Here are some considerations about the appearance of wood treated with these techniques, from a recent paper by Genco et al. [3], which describes similar consolidation agents (epoxy resins and natural / synthetic thermoplastic resins), applied on samples made of different types of wood (poplar and walnut), and degraded by freezing/defreezing cycles. After applying solutions of natural and synthetic resins (both acrylic and aliphatic), the color change is minimum but the level of consolidation is rather low, while the two epoxy resins, Epo Epo Top 155 and Templum, despite the relevant color change, give the best level of consolidation, excellent in the case of Templum Epo Top. So, you have to decide whether color change (as a strengthening of the tone or "wet" effect) is acceptable or not in order to achieve the best performances in mechanical strength. If this parameter is not affecting in the case of the repair of a structural element such as a beam, it is rather discriminating when treating objects of historic and artistic interest.

The second case refers to the use of Epo 155 as compensation material for a statue made with 
papier mache and covered with painted suede representing "Christ dead": the main feature of the work is given by the mobility of his limbs. In fact, to make the object very similar to a real body, the shoulders, the elbows, the wrists, the knees and ankles are bonded to the torso by flexible joints. To better mimic the movements of the human body during the religious processions and the Crucifixion, even the neck and torso are articulated. The artist tried to recreate the theatrical effect making the head reclining down and the bust bent forward, to simulate the state of desolation and weakness of Christ on the Cross. Since the work had been done for the Good Friday procession, it was necessary to restore functionality of the joints, initially made of suede, that degraded over time. To fill the gaps they used Epo 155, thickened with micronized silica to make it spreadable and easy to be colored with light yellow ocher, the pigment that best approach the color of suede. A bridge made of canvas was coated with the mixture and then glued on the borders of the gap with the same epoxy product. In such a way they obtained a level of extension difficult to achieve with other materials (as you can see in the pictures below). Lastly they retouched the work with acrylic paint for better adhesion to the support and greater elasticity of the color.

Except for reversibility, the cross linking epoxy resins again show their outstanding properties.
However, we should take into consideration also the possibility of creating a barrier layer (for example by using a reversible thermoplastic resin such as Paraloid), between the faces to be bonded, or even to increase the effect, filling the porosity of the substrates through applications of cyclododecane. 


References
  • Henriques D.F, Nunes L., de Brito J.; “Test of consolidation products for wood degraded by fungi” 3rd Meeting on pathology and rehabilitation of buildings – PATORREB 2009, Porto, 467-472.
  • Henriques D.F., Nunes L., de Brito J.; "Consolidation of Timber Degraded by Fungi in Buildings: an Experimental Approach Records from the 18th CIB World Building Congress, Salford, UK (2010), 424-433.
  • Genco G., Lo Monaco A., Pelosi C., Picchio R., Santamaria U.; “La valutazione sperimentale dei consolidanti per il legno” Records from XXV Conference “Scienza e Beni Culturali” Conservare e restaurare il legno, Bressanone, (2009), 475-484
  • Panvini R., Nucera G.C., Gabbriellini C., Rossi F., Borgioli L.; “Il Cristo morto di Mazzarino. Un singolare caso di applicazione di resine epossidiche su un’opera polimaterica” Restoration project n.45 (2008).
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