One Tiny Problem with Mortar & Grout: Nanosilver

Melissa Coffin | May 19, 2014 | Newsletter

This blog post, originally shared in the Pharos Signal, includes information about parts of Pharos that are no longer available. Please use it for historical reference and for the other useful information it contains.

When we opened a Pharos product category for Ceramic Tiles last month, the mortars and grouts required to install them were a bit of an afterthought: essentially cement, we assumed, adding these products to Pharos would be a snap.  As it happens, we were wrong on both counts.  Far from just cement, there is a wide range of chemistries used to formulate mortars and grouts, each unique in the ingredients and additives required… except for one tiny exception: our research found that silver- and nanosilver-based antimicrobials are virtually standard issue in mortars and grouts.

Our review of cementitious, epoxy, and urethane mortars and grouts led us through countless technical data sheets, MSDSs, and product brochures, and the majority of grouting products (and some mortars that can also be used as grout) market formulations with antimicrobial/mold resistant/mildew resistant additives.  This makes sense, of course, since grouts are on the surfaces of floors and shower stalls where moisture and resulting microbial growth are bound to be frequent finds.  But, product literature advertises brand-specific antimicrobial protection, never identifying outright the nature of the additive.

Microban was a commonly cited additive in grouts and mortars, but while “Microban antimicrobials” can be synonymous with triclosan, the company engineers some 20 different antimicrobial products based on various chemistries and materials. [1] Further, our research found that triclosan did not perform well in mortars and grouts [2], and so was an unlikely suspect as an additive with such widespread use. 

One material that did perform well in these products was silver.  Silver zeolite, silver nanoparticles, [3, 4] and silver glass are three forms of silver our research found that successfully provide antimicrobial silver ions at the surface layers of grouts where they can retard the growth of mold and mildew.  Microban’s catalog includes silver based technologies, consistent with the product literature we’d reviewed early on.  Further digging into Bostik’s Blockade brand antimicrobial additives revealed that it, too, was a silver-based product by AgION.

So what’s the problem? Silver-based antimicrobials, some of which rely on silver nanoparticles, have a tendency to leach out of the materials into which they are incorporated. 

A 2011 study by the Swedish Chemical Inspectorate found that in clothing treated with silver antimicrobials, as much as 43% of the silver additive had been washed away after just 3 washes, and as much as 98% of the silver was gone after 10 washes. [5, 6] Further, a 2008 study of silver zeolite antimicrobials in cementitious mortar mixtures also found leaching to be common.  Researchers in that study concluded [4],

“The fact that leaching of the ions occurs has important consequences on the biocidal activity of the mortar in the long term. Successive leaching of ions could result in the loss of the biocidal activity. In practice, the life span during which the mortar shows bactericidal activity will depend on environmental parameters such as the type and amount of soiling on the surface and the time during which the surface is kept moist.”

These silver particles wash down drains and enter water treatment facilities.[7]  A 2013 a GreenScreen assessment of nanosilver rated it as “very high” for environmental persistence and aquatic toxicity, [8] but the full scope of what this means to the health of dynamic ecosystems is still unknown [7, 8]

The Swedish Chemical Inspectorate noted as part of its textile study that silver concentrations in sewage sludge in Sweden had been on the decline since the photography industry became digital and moved away from chemical development, but that silver concentrations are starting to accumulate once more [6]:

“The fact that levels of silver are no longer declining in the sludge is assumed to be due to increased use of silver as a biocide in various articles. The use of biocides has increased in a number of consumer products, and in addition to textiles now occurs for example in shoes, refrigerators, toothbrushes, plastic bottles, vacuum cleaner filters, shower curtains, kitchen worktops, mattresses and chopping boards.” 

And grouts.

So, what's a concerned consumer to do? That’s unclear at the moment.

None of the mortar and grout manufacturers that we reached out to responded to our requests for information.  As is our way in these instances, we have researched the products on our radar based on publicly available data, and filled in data gaps with what we’ve determined to be common ingredients in products of their type.  While the grouts displaying in the Pharos Building Product Library vary in VOC emissions, or recycled content, as of today, virtually every product evaluation includes the common use of silver-based antimicrobials.

So, grout formulators of the world, here’s your chance to differentiate your products from others on the market: if you’re not using silver-based additives, tell us about it. 

 

References:

[1] “Microban Technologies” website, accessed 13 May 2014.  Available: http://www.microban.com/microban/microban-technologies

[2] US Patent Application US20070281096A1, Microban Products Company, 2007. 

[3] US Patent US8083851 B2: “Discrete particles of an additive; a wettable, caked inorganic material with a dispersed inorganic or organometallic antimicrobial agent; releasing antimicrobial metal ions in water.”  Sciessent, LLC: 2011

[4] De Muynck W, De Belie N, and W. Verstraete, "Antimicrobial mortar surfaces for the improvement of hygienic conditions." Journal of Applied Microbiology: 108 (2010) 62–72.

[5] Frank, Ulrike.  "Identity, grouping and characterisation of silver based biocidal active substances including nano-silver," Feb 2012.  Slide deck.  Accessed 12 May 2014: http://www.bfr.bund.de/cm/349/identity-grouping-and-characterisation-of-silver-based-biocidal-active-substances-including-nano-silver.pdf

[6] Swedish Chemicals Agency, “Antibacterial substances leaking out with the washing water –analyses of silver,  triclosan and triclocarban in textiles before and after washing.”  2012.  Accessed 13 May 2014. Available: http://www.kemi.se/Documents/Publikationer/Trycksaker/PM/PM1_12_Antibact_eng.pdf

[7] Kim B, Park C, Murayama M Michael F. Hochella Jr, “Discovery and Characterization of Silver Sulfide Nanoparticles in Final Sewage Sludge Products.”  Environmental Science & Technology.  2010, 44 (19), pp 7509–7514.

[8] Natural Resources Defense Council, Powerpoint on GreenScreen and Nanosilver GreenScreen slide deck.  Accessed 15 May 2014.  Available: http://docs.nrdc.org/health/files/hea_13061001b.pdf