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The chemistry involved between detergent and soil is relatively simple. It might seem therefore that a small number of products should be able to cover most cleaning duties; and indeed some products have a broad range of application. However, the varying conditions and situations of cleaning mean that products need to be matched to a large number of variables; consequently product ranges can be quite extensive and it takes experience and knowledge to best match a product to the cleaning application.
It is important to remember foam does not clean. Foam is merely the delivery vehicle which delivers the chemicals, that have cleaning functionality, to a surface. Similarly, a Gel is merely a delivery agent to deliver cleaning chemicals to a surface and ensure long contact time.
Foams gradually breakdown on a soiled surface releasing the detergent solution to the surface over a period of time. Generally, the longer the cling the longer the soil will be in contact with fresh detergent solution; however ultimate performance depends on selecting a product that delivers the correct chemical energy.
Gel technology is not so relevant in most open plant cleaning situations; however there is still a place for gels in niche applications.
Alkaline products generally clean by saponifying fatty materials and emulsifying the resulting modified soil so that it can be suspended in solution. Potassium hydroxide produces more soluble saponification products than sodium hydroxide; thus, they are easier to rinse away and are less likely to redeposit. Potassium hydroxide is however considerably more expensive than sodium hydroxide.
Alkaline products will not effectively remove protein films such as those that build up in abattoirs or on meat slicers. For these applications, a Chlorinated product is preferred.
Alkaline products will always cause water hardness to precipitate as a scale. To prevent this, products in this category contain scale control agents. It is important to always identify the hardness of the water used to dilute the foam concentrate and then match the final product concentration to the water hardness. Failure to do this will result in scale formation on surfaces.
As a rule, the scale control agents in alkaline products will not dissolve mineral scales, for this an acidic product is required. The scale control agents will interact with calcium and magnesium compounds in soils to aid breakup, but for this to be effective it is essential to use a product concentration above the minimum concentration required for water hardness control.
Caustics based products are used where the soil is difficult to remove (carbonised or polymerised) or where we need a large chemical energy because the physical energy input and or the time is low. Caustic products are used in most CIP systems, as the detergent in tray or crate washers and for boiling or soak cleaning of cookers or fryers. The addition of Adhol 10 to caustic solutions creates oxygen release and turbulence in the solution thus aiding soil removal; this process needs careful control to avoid ‘boiling over’ of hot caustic solutions. Caustic foam products are used on heavy soiling such as ovens.
Much that has been said for alkaline foams also applies to caustic foams. Sodium hydroxide, potassium hydroxide and metasilicate are the main sources of alkalinity.
Caustic products generally clean by hydrolysing organic soils as well as saponifying fatty materials. Caustic products will not effectively remove protein films, such as those that build up in abattoirs or on meat slicers. For these applications, a chlorinated product is preferred. Caustic products will always cause water hardness to precipitate as a scale. To prevent this, products in this category contain scale control agents. It is important to always identify the hardness of the water used to dilute the product and then match the final product concentration to the water hardness.
As a rule, the scale control agents in caustic products will not dissolve mineral scales, for this an acidic product is required. However, the scale control agents will interact with calcium and magnesium compounds in soils to aid breakup, but for this to be effective it is essential to use a product concentration above the minimum concentration required for water hardness control.
Whereas alkaline and caustic detergents generally clean by saponifying fatty materials/soils and emulsifying the resulting modified soil so that it can be suspended in solution; chlorinated detergents have the additional chemical advantage of oxidation. Here, the hypochlorite component of the formulation can break up large molecules (proteins, colourings, flavourings) into smaller molecules that can then be more easily removed from surfaces and held in solution by emulsification and rinsed away.
Chlorinated alkali detergents will always cause dissolved water hardness minerals to precipitate and deposit as a scale. To prevent this, products in this category contain scale control agents. However, unlike in alkali and caustic detergents, EDTA type chelants cannot be used in chlorinated products. Instead, we must rely on threshold effects for control. The effectiveness of the threshold system can depend on the in-use strength of the detergent. In some circumstances, it is this that will dictate dosing levels. The scale control agents in chlorinated detergents do not dissolve scale soils. They only suspend them in solution, allowing them to be rinsed away. If chlorinated foams are used in the region of 3 – 5% v/v they will be compatible with most waters in the UK and Ireland. If products are used below 3%, instability will almost certainly occur in all but the softest of waters (typically Scotland). The scale control agents in chlorinated products will not dissolve mineral scales, for this an acidic product is required.
It is advisable to keep maximum in-use temperatures to 50°C ; above 50°C pitting corrosion of stainless steel will start to occur. Never mix chlorinated foams with acid foams, this will produce toxic chlorine gas and never mix chlorinated foams with non-chlorinated alkaline or caustic foams, this can create a violent explosive reaction.
Acids are primarily selected for their ability to remove mineral scale and protein deposits. Acids will not remove fatty or greasy soils. Bear in mind that during water scale removal, CO2 gas is naturally evolved. It is advisable to ensure that systems are suitably vented.
Acidic foams will only dissolve minerals. They will be ineffective against other soils, unless those soils are being held together by inorganic components. This is sometimes the case as with lime soaps or protein scales, where calcium has become chemically associated with the soil. In these instances, acids will be effective; but their typical use is to remove inorganic mineral scales.
Acids are consumed and become depleted during use. This obviously has an impact on the time input. In other words, it is pointless leaving an acid to work for one hour, when all the chemical energy available has been used up within fifteen minutes. Monitoring acid strength during a descaling operation with a test kit is essential, once product strength has fallen to 50% of the starting value, it should be topped up with additional acid.
Conductivity cannot be used to monitor the fall in acid strength because the break down products formed by dissolution of scale and neutralisation of acid will have conductivity little different to the starting acid.
Neutral Detergents are suitable for hand use in sinks or soak applications. All have very high emulsification and wetting properties. Because the products are caustic free there is no saponification of fats; although this is a disadvantage, the products are compatible with all water hardness conditions.
The distinction between alkaline and caustic products is complex. In the past, it could be related to irritant and corrosive classification; but under CLP classifications this no longer holds true.
Sodium and potassium hydroxide are both sources of alkalinity and at high strength referred to as caustics; but many products that have low levels of sodium and potassium hydroxide tend to be referred to as alkaline.
In addition to sodium and potassium hydroxide, there are many other sources of alkalinity, for instance: EDTA in its neutralised form will result in pH values greater than 7 and in sufficient concentrations will give pH values in excess of 11. Materials such as sodium meta silicate also give very high pH values and can be considered as alkaline, but it is a form of alkalinity very different to that given by sodium and potassium hydroxide.
Sodium hydroxide (NaOH), caustic, is manufactured by the electrolysis of brine solutions. Until very recently, this was performed using a mercury electrode, the result was that sodium hydroxide always contained trace levels of mercury. Modern production processes are mercury free and typically produce sodium hydroxide as a solution of 50% wt./wt. concentration.
50% wt./wt. solutions of sodium hydroxide have SG values of 1.52 @ 20 degrees C, this means that solutions are very heavy. Furthermore, the freezing point of a 50% wt./wt. solution of NaOH is very high, making it difficult to use and deliver in the winter, unless tanks and pipes are trace heated and lagged.
There are several ways of expressing the concentration of caustic products, these can be confusing. It is important to know how to convert between the various expressions.
This describes the weight of NaOH dissolved in a weight of water or formulated product. For instance, 500g NaOH/1000g of total solution is a 50% wt./wt. solution.
The strength of caustic cleaning solutions is often expressed as % wt./v NaOH; this is commonly referred to as causticity. This describes the weight of NaOH dissolved in a volume of water or formulated product. For instance: 20g of NaOH/1000ml of solution is a 2% wt./v solution.
The relationship between wt./wt. and wt./v is Specific Gravity (SG).
For example: 30% wt./wt. NaOH (Caustak 30) has an SG of 1.3. To express this to a wt./v it is necessary to multiple by the SG. 30% wt./wt. NaOH x 1.3 = 39% wt./v NaOH
This term expresses the volume of product diluted in water. For instance: 20ml of Caustak 30 made up to 1000ml can be expressed as a 2% v/v solution.
To calculate the % wt./v NaOH in a 2% v/v solution: 39 x 2/100 = 0.78% wt./v NaOH
Whereas it is relatively easy to compare the strengths of different caustic products, it is difficult to compare the strengths of acid products, because different acids have different dissociation constants, so for example citric would be considered a weak acid and nitric acid would be considered a strong acid, regardless of the actual % wt./wt. strength. The term ‘Acid Value’ is useful to compare the strength of different acids in terms of their ability to neutralise potassium hydroxide.
The units of Acid Value are: - mg KOH/g. This describes the milligrams of potassium hydroxide required to neutralise 1g of acid detergent. This value is useful as it intrinsically considers if an acid is Mono, Di or Tribasic and is blind to dissociation constants. In simple terms, the higher the Acid Value, the more scale that a given weight of acid will dissolve.
In combination with chlorinated products, all acids will produce toxic chlorine gas. When using acids care should be taken to ensure that there are no common drains that are receiving chlorinated product from other parts of a site.