Wednesday, August 31, 2016

The Advantages of Epoxy Resin versus Polyester In Marbles & Stones



The Advantages of Epoxy Resin versus Polyester
In Marbles & Stones


Introduction

In any high-tech structural application, where strength, stiffness, durability and light weight are required, epoxy resins are seen as the minimum standard of performance for the matrix of the composite. This is why in stone and structural applications, epoxies have been the norm for years. However 95% of stones today are still processed with polyester resin. The main consideration for materials selection for most stone processors is cost, with performance and more importantly value for money often being a secondary consideration. As a general rule epoxy resins are twice as expensive as vinyl ester resins and vinyl ester resins are twice as expensive as polyesters. Since the resin can constitute 40 to 50% of the weight of a composite component, this price difference is seen as having a significant impact on the cost of the substrate.

However, when considered against the cost of the whole substrate (Stone) the cost is relatively insignificant, and the value of higher quality and long term gain of better durability (therefore better resale value) can be tremendous.

What contributes to this better value…..?

Epoxy resins have performance advantages over polyester and vinyl esters in five major areas:

  1. Better adhesive properties due to better penetration (the ability to permeate and bond to the reinforcement or core)
  2. Superior mechanical properties (particularly strength and stiffness)
  3. Improved resistance to fatigue and micro cracking
  4. Reduced degradation from water ingress (diminution of properties due to water penetration)
  5. Increased resistance to osmosis (surface degradation due to water permeability)

Adhesive Properties

Epoxy resins have far better adhesive properties than polyester and vinyl ester resins. However many times have you known a polyester putty marble filler peel out during polishing? The superior adhesion of epoxy is due to two main reasons. The first is at the molecular level, where the presence of polar hydroxyl and ether groups improves adhesion. The second is at the physical level - as epoxies cure with low shrinkage, the various surface contacts set up between the liquid resin and the reinforcement are not disturbed during cure. The result is a more homogenous bond between fibers and resin and a better transfer of load between the different components of the matrix.

High adhesion is especially important in resistance to micro-cracking (see later) and when used as filling. The bond between the core and the top layer is usually the weakest link of the substrate, and the superior adhesive properties of the epoxy resin greatly increase the strength of the interface between skins and core.
 
Mechanical Properties

Two important mechanical properties of any resin systems are its tensile strength and stiffness. The figure below shows results of tests carried out on commercially available polyester, vinyl ester and epoxy resin systems, either cured at room temperature or post cured at 175°F.

After a cure period of seven days it can be seen that the tensile strength of the epoxy resin is 20 to 30% higher than those of polyester and vinyl ester. More importantly, after post cure the difference becomes ever greater. It is to be noted that stones processed with polyester resins are rarely post cured in the workshop while stones processed with epoxy quite often are. However, in practice all stones can quite often see “natural” post cures.

The consequences are twofold:

Structurally

A post-cured epoxy laminate will exhibit tensile strength and modulus (stiffness) close to double that of a non-post cured polyester or vinyl ester laminate.

Cosmetically

Polyester and vinyl ester resins shrink up to 7% volumetrically and because the resin continues to cure over long periods of time this effect may not be immediately obvious. This cure accounts for the print through effect observed on a lot of older polyester processed composites. In comparison, epoxies shrink less than 2% and an epoxy laminate will be a lot more stable and have better cosmetics over a long period of time than a polyester one.


 Fatigue Resistance and Micro-Cracking



In most cases a properly processed laminate will never be subjected to its ultimate strength so physical properties of the resin matrix, although important, are not the only criteria on which a selection has to be made. Long before ultimate load is reached and failure occurs, the laminate will reach a stress level where the resin will begin to crack away from those fiber reinforcements not aligned with the applied load. This is known as ‘transverse micro-cracking’ and although the laminate has not completely failed at this point, the breakdown process has commenced.

The strain that a laminate can take before micro cracking depends strongly on the toughness and adhesive properties of the resin system. For relatively more brittle resin systems, such as many types of polyester, this point occurs a long way before laminate failure, and so severely limits the strains to which such laminates can be subjected. In an environment such as water or moist air, the micro-cracked laminate will absorb considerably more water than an un-cracked laminate. This will then lead to an increase in weight, moisture attack on the resin and fiber sizing agents, loss of stiffness and with time, and an eventual drop in ultimate properties.

The superior ability to withstand cyclic loading is an essential advantage of epoxies vs. polyester resins. This is one of the main reason epoxies are chosen almost exclusively for most of the structures.








 
Degradation from Water Penetration

An important property of any resin, particularly in during rainy and winter seasons, is its ability to withstand degradation from water penetration. All resins will absorb some moisture, adding to a laminate’s weight, but what is more significant is how the absorbed water affects the resin and resin/fiber bond in a laminate, leading to a gradual and long-term loss in mechanical properties.

Both polyester and vinyl ester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures. As a result, a thin polyester laminate can be expected to retain only 65% of its inter-laminar shear strength after immersion over period of one year, whereas an epoxy laminate immersed for the same period will retain around 90%.

Osmosis

All laminates in a marine environment will permit very low quantities of water to pass through them in vapor form. As this water passes through, it reacts with any hydrolysable components inside the laminate to form tiny cells of concentrated solution. Under the osmotic pressure generated, more water is then drawn through semi permeable membrane provided by the gel coat in an attempt to dilute this solution. This water increases the fluid pressure in the cell. Eventually the pressure will distort or burst the gel coat, leading to a characteristic “chicken-pox” surface.
To delay the onset of osmosis, it is necessary to use a resin that has both a low water transmission rate and a high resistance to attack by water. A polymer chain having epoxy linkages in its backbone is substantially better than polyester or vinyl ester systems at resisting the effects of water.




Six side Sealing of stone – Breaking myths of Debonding



Six side Sealing of stone – Breaking myths of Debonding

For years the use of sealers has been expressly for the protection of the stone or tile surface. Most, if not all, adhesive companies warranty their adhesives only if the back of the stone is free from any sealer as sealers are seen as “bond breakers” adversely affecting the integrity of the adhesives ability to bond to the stone or tile surface. However this situation has meant that many of the problems created by water absorption through the back and the sides of stone have gone unresolved. New and current technology now offers sealers that can successfully be applied to the bonded surfaces of stone without becoming bond breakers. To look at these and how they work I firstly want to investigate the problems and issues relating to porous stone and specifically water absorption through the sides and back.

Picture framing, efflorescence, soluble mineral contamination (for example iron Sulphides such as Pyrite) and prolonged water marking are some of the problems created when water is absorbed by the back and sides of some types of stone. The mechanics are as follows. When a stone is installed over a concrete substrate the concrete can contain potential soluble contaminants such as salts and other minerals. The underlying cement based screed or topping as well as the cement based adhesive and grout also have the potential to hold some of these contaminants. In most cases the contaminants will not react unless there is water present. Water is both the catalyst as well as the transport mechanism. The initial and most important source of water that triggers much of the reaction originates from the hydrating adhesive or mortar bed that is even more aggressive due to its high ph. With water the soluble minerals travel to the surface by way of evaporation and capillary action working their way through the stone and grout. In many cases the grout is more porous having higher water absorption than the stone creating an easier exit for the evaporating soluble minerals. This explains why in many cases the resulting stains are revealed as picture framing or at least concentrated around the sides of the stone and grout joint. Once on or near the surface the contaminants further react with the increasing rate of oxygen and ambient air temperature to form various compounds or simply evaporate or dissolve only partially leaving behind the unwanted stain or compound.



A good example of this mechanism at work is the soluble iron salts found in the granites and marbles across various projects.

The water from the thick mortar bed under the stone once absorbed into the stone body easily reacted with the soluble salts to form highly visible iron blooms. In some cases the iron salts would turn the complete stone a light shade of yellow. The solution to this problem is simple – if the stone’s natural water absorption could be reduced close to zero then the risk of iron contamination would be similarly reduced. The best and most cost effective way to reduce the stones water absorption is to seal the stone on all six sides. 

We have all known for many years that the trick to managing many of the water related problems of soluble mineral staining such as the iron salts is to lower the stones natural water absorption by sealing it on all six sides while still maintaining good vapor transmission. (The ability for the sealer to breath is very important as any trapped water can create other issues such as surface debonding by way of excessive moisture expansion. However the formation of the contaminants is not only due to the presence of water but also the quantity of water and rate of evaporation. If the amount of water is reduced and the rate of evaporation high enough that the water does not condense then the soluble minerals will also exhaust through the surface rather than solidify ). The problem however is that most sealers either did a poor job of repelling water in a high alkaline environment as that found at the interface between stone and cement mortar, or reduced the bond strength of the adhesive system. The latter is the reason why most adhesive manufacturers only warranted their adhesives when applied to clean unsealed stone. This claim in turn made clients reject any sealer solution to the problem as well as making sealer manufacturers uninterested in developing specific sealer technology. However as stone’s use increased globally so did the problems related to the high water absorption and chemistry of certain stones. All of this at last led to the development of sealers that could in fact both reduce a stone’s water absorption as well as maintaining the adhesive’s bond.

RachTR has designed the sealer specifically for application to the back and sides of stone called RachTR Back Seal. It is designed to hit the main market requirements for such a product – low cost per m2 (or sq ft), highly water repellent, good vapour transmission and of course not being a bond breaker for the adhesive. However we realized that the best way to apply sealers to the sides and back in a cost effective manner was to dip the entire stone. However this presented a problem in that many clients wanted a low cost sealer for the back and sides such as RachTR Back Seal but wanted a premium product for the actual surface, which would be exposed to long-term dirt and contamination. Therefore any specialized back applied sealer had not only to be compatible with a premium sealer but also needed to allow the premium sealer to penetrate it so the correct quantity of premium sealer could be applied to the surface. RachTR Top Seal is designed to satisfy both these conditions. The sub surface sealers will penetrate right through the RachTR Back Seal enabling the complete and correct quantity of premium sealer to be applied guaranteeing the long term performance of the final sealer.

Both of these sealers are tested regarding shear bond to ensure they do not act as bond breakers.

The contemporary existence of sealers that can be applied to the sides and back of a stone or tile now help to manage and greatly reduce the risk of the long endured problems created by moisture moving through the stone especially during installation and the process of final cure. The argument by both clients and adhesive manufacturers to not seal all sides due to the possibility of the sealer being a bond breaker is no longer valid now the technology exits to do so. 

Using a suitable sealer on all six sides is part of a total water management system that should be implemented to fully control the uses around leaching of soluble minerals. These include for example the use, where appropriate, of waterproof membranes, epoxy remediation systems, proper falls, factory prepared adhesives and grouts etc.