End User Focus: Sticking to tradition

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Published: Tuesday, 23 August 2011

These days, tile adhesives are a complicated mixture of additives, chemicals and fillers, each playing a crucial part in the adhesive performance. But, as Ed Hiam explains, traditional materials such as industrial minerals still play a key role in this evolving market

 
You could be forgiven for thinking that a ‘glue’ used for sticking tiles to a wall or floor in your common-place bathroom or kitchen is a simple affair. But over recent years these products have become very complex, with an increasing array of natural but well-processed industrial minerals being coupled with a huge selection of man-made additives. This article will not only look at what is contained in tile adhesive to make it work, but also the minerals that are used along the way in making other components.

Tile adhesives are available in many forms but most commonly, and firstly for cost, as a single component - bagged powder - that is reconstituted with water before use. Then for convenience as their direct opposite, adhesives can be made in the form of a ready-mixed paste that is used straight out of the bucket.

Secondly, there are two component tile adhesives that rely on the mixing of the bagged powder either with a liquid (that will contain an emulsion polymer, as opposed to the more recent trend of redispersable powder polymers concealed within bagged-powder tile adhesives), or with two pastes such as an epoxy resin with its associated hardener. This latter type is not so common as these grades are high performance and high cost but, most often than not, they double up as the grout between the tiles as well as being the adhesive under the tile.

The challenge that the construction product manufacturing industry faces is, in effect, keeping up with the fashion of decorating the home or workplace with an ever increasing choice of finished surfaces. Tiling represents something that is tough, durable and hygienic, but also very personal and pleasing to the eye. Combine this finish with under-tile or under-floor heating it can also provide a surprisingly warm surface at a modest cost which can provide background warmth even in large open spaces.

To this effect the tile itself can be natural stone such as limestone or sandstone flags, thinly sliced marble that is virtually transparent, mosaics that are kept in a pattern by a mesh backing through to ceramic tiles that are fired or vitrified with or without a glaze. The size of the tile, from tiny mosaics to quarter square-metre giants or bigger, places an immediate demand on a tile adhesive as a ‘hung’ weight if fixing vertically. The opposite problem on a floor can be the ability to physically bed or lay the tile with sufficient hand pressure to achieve a satisfactory bond to the ground surface.

Market influences

Market influences on tile adhesives includes standards - to the manufacturer, the technical ability of the adhesive can feel like a set of constantly moving goal posts. The ratification of individual country’s standards into a harmonised European Standard or ‘EU norm’ has created frustration (and cost!) along the way.

The ultimate aim starting from the development chemist to the end user is to have a grade or classification of use on the packaging that takes the worry out of selecting the right adhesive to the right tile, to the right surface / substrate. This can achieve a CE mark for the manufacturer, but some have rejected this and choose instead a competent local workforce / approved applicator to get it right.

Fashion
keeps us on our toes and never ceases to place new demands on tile adhesive formulae. The overall move in the market has been towards larger tiles, less porous and better glazed tiles, and surfaces on which you can put them. It is not uncommon now to see outside tiling in the UK, even onto wooden surfaces that can develop wide ranging stresses due to moisture movement. Flats and high rise living accommodation can put demands on thermal and acoustic performance of tile adhesives too.

Procurement
is key to a cost-effective tile adhesive via the keen sourcing of raw materials. Currency exchange rates for importing, haulage costs for miles travelled, and free competition without monopoly supplies are challenges that have to be met not just during the development process but also as ongoing maintenance throughout the life of the product.

Environmental
considerations are also a market influence, with the industry learning the hard way that disposing of inert or neutralised waste is a lot cheaper than paying for hazardous disposal. To this effect powder waste has to be mixed and left to ‘go hard’ whereas ready mix paste waste will never go off unless you take it out of the bucket and spread it out to dry.

A particular hazard if not treated with respect and personal protective equipment is two component tile adhesives such as epoxy resin, where skin sensitivity can develop from prolonged exposure and even after mixing the two components together there is the issue of ‘ullage’, ie. the residue in the tin you can never quite get out!

It is worth mentioning here that industrial minerals can help with this in the case of powder tile adhesives that contain Portland cement. A risk of contact dermatitis exists from exposure to hexavalent chromium ions. These can be reduced to the non-sensitising trivalent chromium ions for a limited time period (an ongoing debate) by the addition of, in order of popularity, ferrous sulphate, stannous sulphate, manganese sulphate and stannous chloride. These are added by the cement manufacturer and/or by the tile adhesive manufacturer depending on the shelf life expected from the finished product.

Typical formulae

A bagged powder tile adhesive can be a simple formula with typically five or six raw materials in its blend, which contrasts with a ready-mixed paste formula that can be as complex as 10 to 20 raw materials. Formulations across the market developed by construction product manufacturers follow their own ‘fan base’. This is a two-way process involving praise and complaint, where people can be fickle about what material they like and what material they do not.

A contractor who may be laying several hundred tiles at a time is aware of both speed and comfort; it is to this end then that formulations become confidential and sales well-earned between respective manufacturers.

Based on readily-available starting formulae obtained from raw material suppliers, principally the ones supplying the binding medium, discussed below are the typical formulae and terminology used by the market. Note that as we discuss the technical aspects of why one industrial mineral is better than another, I will name what property is desired followed by the type of mineral required.

An example could be the difference between a mineral’s respective oil absorption value and particle size distribution, therefore its demand on the gauging water or liquid polymer emulsion it is going to be mixed with. When a tile adhesive has cured and is exposed to the working service conditions, the ability to resist water absorption would have an influence on wet/dry cycling, or even its ability to help the tile resist steam cleaning.

In recent years with an increase in environmental awareness, another example of mineral choice could be just about the ‘eco tick in the box’. Avoiding the relatively small charge of aggregate levy from a marketing stance might not seem like much compared to the benefit of claiming a percentage recycled content under the government guidance of creating a sustainable construction product directory. This is where consumers of tile adhesives, such as architects, can have some degree of ‘green’ reassurance in their choice.

There are minerals that have an effect on colour, and while you could be forgiven for wondering why would this matter when you are going to be looking at the tile and the not the adhesive, it should be appreciated that the cost of a tile can be significant - and customers do not want the adhesive grinning through and spoiling the tile colour.

The binder

This is the key functional constituent of a typical formulation and is responsible for many properties including the overall working consistency or handling characteristics, the stick and the hardness of the cured tile adhesive.

The most common form of binder in powder tile adhesive, given its cost versus performance is, ordinary Portland cement (OPC), which hydrates with the addition of water to form a material that cures and gains strength within days. This was improved under the old cement classification system with a finer milled version known as rapid hardening Portland cement (RHPC). These are typically referred to under the latest Euro classification as 42.5N and 52.5N respectively, referring to the typical strength attainable at 28 days in standardised tests. This is still the binder of choice in emerging tile markets like India and China.

As time is money in the contracting game, there has been a trend towards faster-setting tile adhesives where the ability to fix and grout within a day is not as surprising or as uncommon as you would think! In order to satisfy this trend over the last 10-15 years in particular, firstly the use of calcium aluminate cement (CAC) and then secondly calcium sulpho-aluminate cement (CSAC) have been developed and employed to speed up the set. It is now possible to bed a tile and to be walking on it within two hours (given a good ambient temperature of 20¡C or so).

The key industrial mineral here has shifted from limestone and shale deposits to bauxite. Simply put, it has been a race to secure bauxite deposits in China that are as white and as pure as possible. Long-term reserves across China and other emerging countries are crucial to this market.

The binder of choice in paste tile adhesives is man-made polymer emulsions. To produce these in a reactor vessel can require a liquid cocktail of 10-15 raw materials. Much of this does not require industrial minerals until such time as you try to convert the liquid emulsion into a redispersable powder by spray-drying.

In this process, a finely atomised spray of emulsion is aimed at a steel disc the size of a dinner plate spinning at approximately 15,000rpm. This scatters the liquid over a wide area where it drops down under gravity as a cloud inside a large sealed chamber the size of approximately three large houses. The emulsion is met on the way down by rising hot air, and gradually hardens into a dry powder as the chamber base approaches.

The ability to identify the right spray-drying additives in the liquid emulsion to assist with the spray-drying process, and also the post-spray minerals that stop this powder from coagulating, is an engineering marvel that takes years to perfect. Here, industrial minerals that offer high surface area, such as precipitated silicas and zeolites for absorbency, are much in demand.

The fillers

This is where the bulk of the industrial minerals are found and are used to keep the overall cost of the formula down by extending the binder as it fills out the mix and provide volume or yield. They are used at an approximate ratio of two thirds filler to one part binder, and this relationship varies depending on the cost versus performance requirements of the finished product.

Minerals are also the most unappreciated part of a tile adhesive formula coupled with neglect over time. This can have dire consequences on product consistency when the properties of industrial minerals drift in and out of colour, oil absorption or grading specification as a source moves through a quarry or deposit. In reality, especially for paste tile adhesives, the effort put in here can be considerable over many months or even years. Getting it wrong here can result in vast quantities of paste that is not easy to recover or dispose of!

Whiteness or brightness
is somewhat of a ‘holy grail’ to the industrial chemist, whether it is a powder tile adhesive that requires white or low colour sand, to chalk or silica flours used in ready mix paste. Geographically, the UK struggles for these sources compared to Spain or France. The ability to source at a keen price a material that will promote whiteness in a formulation, either because it is white or low enough in colour to be enhanced by addition of another mineral (or even the whiteness of the binder), is key to a large part of the formula costing.

Typically over the years the starting choice has been calcium carbonate, either mined or in recent years as the cost has come down, precipitated. This was then backed up by small amounts of titanium dioxide (TiO2) to improve the overall whiteness of the paste. There is today a vast array of minerals that are processed for use as TiO2 substitutes, all on a theme around calcium carbonate, talc or even spar.

One of the pitfalls of these quarried materials is any trace contamination from other minerals; sometimes powder fillers may look white until they wet out, where it reveals a faint undertone of grey or pink - for example from clay contamination. White fillers may also look white until they wet out, which can decrease brightness because of semi-transparency! The physical properties of a mineral need to be understood - not just on their own but also in your formulation.

Workability or consistency
can be quite personal with the effort required by the end user to apply the tile adhesive making or breaking its reputation. There was a time when silica sand was king, but as time has moved on so the aggregate levy tax has come into play. While this is not much in the grand scheme of things it does start to focus the mind on the mileage travelled and the fact that haulage prices have a great influence on low margin commodities. There are only so many geographical pockets of deposited sand around the UK which are rounded in character, compared to quarrying and crushing to produce angular material that does not lend itself as well.

Density
is important, as while tiles are getting heavier the expectation on the tile adhesive is to get lighter. This is relevant to walls where the overall weight loading per square metre can favour the bigger tile without overloading the plasterboard and stud partition for example. Exfoliated vermiculite, steamed pumice and perlite are all available to help with this challenge.

Flexibility
may seem an odd requirement but when you are bedding a tile onto a surface or substrate that has a risk of movement, such as timber, the ability of the tile adhesive to accommodate this without transferring the stress to the tile and cracking it becomes very important (if only nature could make flexible aggregate!). The closest we have is substituting the rigid aggregate, such as the silica sand (and some of the fillers), with natural cork processed into various grades of crumb. However, due to the cost and demand of tile adhesive generally, then the next best substitute to keep up with this is rubber crumb processed from lorry tyres. This is a good example of ‘recyclates’ being used in a very technical way - as rubber sources can also come from the seals of washing machines and double glazed window frames.

On the subject of recycling and as a material that consumes vast quantities of silica sand for making containers and windows, glass deserves a mention. In today’s UK market there is now a (some would say long overdue) shift towards a can-do attitude in producing and finding routes to market for recyclates. The health and safety benefit of fine flours or powder fillers produced from the grinding of recycled glass provides a non-silicosis risk product compared to the historic practice of milling silica sand. Virtually any sand grade can be matched from bottle glass cullet or as a low colour option from the clear flat glass of windows. There is no excuse not to use some somewhere in your formula! Also, the chemical resistance, stain resistance and cleanliness of an epoxy tile adhesive and grout can be vastly improved when switching from carbonate to silica fillers.

The additives

Apart from being easy to spread and apply, the main function of a tile adhesive is take hold of a tile and never let go! The ability to grab the tile and hold onto it securely is the responsibility of engineered additives in the form of swelling polymers with long tangling backbones and sticky side chains. Firstly these are cellulose ether derivatives from the wood and paper processing industries, supplied as a powder. Secondly, these are often but not always (due to cost) coupled with a polymer such as acrylic, which - depending on whether you are making a paste or powder tile adhesive - will be supplied as a high solids emulsion or its redispersable powder-polymer counterpart, respectively.

Anti slump or sag resistance
is of particular importance to hanging large tiles on a wall. The tiling trend is towards bigger tiles, which means more weight and a higher risk of them slipping. Clays in their simple forms have evolved considerably over recent years. Gone are the days of just adding a bit of kaolin for whiteness and bentonite for a bit of thixotropy. There are now sources of attapulgite, smectite, and sepiolite clays from all over the world that are engineered to provide high structuring character in the form of a high volume swell. They can even act as a partial replacement for the cellulose thickeners discussed above.

Drying time
may sound a little odd, but the clever promotion of a certain type of cement hydration by combining CAC or CSACs with sources of calcium sulphate (as gypsum, Plaster of Paris or anhydrite) enables the gauging water (used to reconstitute the powder tile adhesive) to be locked in quickly. Not only is rapid set promoted, but rapid strength gain as well, giving a quick return-to-service time for grouting.

Setting time
is important as the ability to use a powder tile adhesive in winter requires good acceleration of the cement components by calcium formate, lithium carbonate or sodium carbonate. The opposite is true when plenty of working time is needed in the summer and then citric acid, tartaric acid, or tri-sodium citrate is utilised. The clever bit comes into play when you have both in a formula that are balanced against each other to fine tune the desired set time. There are many other additives that could be discussed, but it is a long and confidential tale.

The future

Tile adhesive consumption is set to increase as tiling gains popularity in the UK and worldwide. It is not just confined to small areas in bathrooms and kitchens now, as the whole living space of a house can be targeted, including the patio and garden. Formulations and standards will adapt to cope with this as the construction industry is poised for recovery out of the recession, and the demand for new housing far exceeds supply. There is also strong demand in the refurbishment/home improvement market from people that can no longer afford to move.

The formulators of tile adhesives, to name but one type of construction product, must remain focused but receptive to new and alternative mineral sources. Sustainability of existing sources and the increased use of recyclates are an issue that cannot be ignored anymore. One thing is certain in life... taxes - as long as it’s not a tile tax!

Contributor:
Ed Hiam, Hi-Tec Specialities Ltd, UK. Hiam has been in the construction product manufacturing industry for 23 years, filling many roles in what is now termed the ‘drymix’ industry. He has principally worked as a development chemist but has also been involved in QC and production management, customer service, site support, sales and consulting. Hiam is a keen follower of associated trade bodies and maintains a mentoring philosophy with academia.