CLEANING AND DEGREASING

What is cleaning?

Cleaning is nothing more than the displacement of dirt from a place where it is disturbing to a place where it is not disturbing, for example from a product to a washing agent.

With cleaning, all dirt is removed that has a disadvantageous effect.

What is degreasing?

With degreasing, only the organic contamination is removed, such as oils and greases. Together with the oil or the grease, the attached dust particles are also removed. The degree to which this happens depends strongly on the degreasing method used.

Purpose of cleaning

The purpose of cleaning can be to give the products a visually attractive (cosmetic) appearance. This class of cleaning is mostly not critical.

Often, however, functional requirements are placed on the cleanness of products that are required for a following operation.

Examples of this are:- assembly in cleanrooms - application of electroplating layers - anodising or chromium plating of aluminium - phosphorating of steel - application of lacquers - heat treatments.

What is clean?

In a production process, the cleanness is often described as so clean that the following processes are not disturbed.

Some simple methods that can be used are: - visual observation - weighing - water refraction test - contact angle measurement - copper sulphate test

Steps in the cleaning process

- pre-cleaning - intermediate cleaning - fine cleaning - final cleaning - rinsing - drying.

In many cases, it is necessary that a large part of the pollution is first removed by pre-cleaning, after which fine or final cleaning takes place.

Pre-cleaning

If products must be stored for a certain time before they are cleaned according to the requirements, manual pre-cleaning is often applied.

Pre-cleaning prevents the dirt from drying on the product, because it is then very difficult to remove.

The requirements that are placed on pre-cleaning are not as high as those placed on fine or final cleaning.

Intermediate cleaning

In a series of mechanical operations, products sometimes have to be cleaned before the following operational step. The requirements on intermediate cleaning are not high. The products must be sufficiently clean to be able to do the following operational step.

Fine cleaning

High requirements are placed on fine cleaning. After this, processes often follow that demand the highest cleanness, such as gluing, lacquering, soldering, welding or electroplating.

Final cleaning

The highest requirements are placed on final cleaning. Examples are reflective metal surfaces (cosmetic) or printed circuit boards for military purposes that must satisfy the heaviest specifications.

With the exception of assembly, mostly no more operations are carried out after final cleaning.

Rinsing The intention of rinsing in a rinsing tray is that the attached cleaning liquid is divided through the whole amount of rinsing liquid, so that a strong dilution is obtained. Movement of the products in the bath helps here. After the rinsing, we no longer have a pure rinsing liquid, but a strongly diluted cleaning liquid. This diluted cleaning liquid is also on the cleaned products. If necessary, the rinsing must be repeated.

If one has a rinsing system with several compartments after each other, we speak of a cascade system. Here, the baths flow over into each other. The result is that much less rinsing liquid is required.

Drying With the use of aqueous cleaning agents, drying must be carried out as an extra operational step after rinsing.The least expensive manner of drying is when a product is heated by a previous operational step, for example rinsing in hot water. If this is not the case, blowing with cold air or with nitrogen can dry the product. Here, the liquid present on the product surface is blown out of blind holes and seams.

Blowing with hot air, drying in a drying oven or with infra-red is more effective, whereby the drying effect is obtained more through evaporation.

Another method is with the aid of water repellent or soluble liquids. Vacuum drying is also applied. As a result of the pressure drop, the vapour pressure drops so that water or solvent evaporates earlier.This process is accelerated when it is done at a higher temperature.

Factors that have an effect on the cleaning process

- chemistry - time - temperature

- movement (agitation).

Cleaning agents

Organic solvents Organic solvents can be subdivided into combustible and (mostly) non-combustible solvents Mineral oil products such as petroleum ether, paraffin and white spirit belong to the group of combustible solvents.

Other organic solvents are acetone and alcohols. These solvents are used for cold degreasing.

Cold degreasing is used for degreasing on a small scale, such as manual cleaning with a cloth or cleaning products in a component cleaner.

Hot degreasing with solvents is used in vapour degreasers. Here, the objects that must be degreased are hung in a solvent vapour.

Aqueous cleaning agents

Aqueous cleaning agents must be subdivided into three groups on the basis of their acid content, i.e.:- acid - neutral - alkaline.

Neutral and alkaline cleaning agents are especially used for degreasing. Aqueous cleaning agents are preferably used at increased temper ature.

Cleaning method

The following methods are often used:

- manual - immersion - spraying - high pressure cleaning - vapour degreasing.

Manual

Manual cleaning is done almost solely with solvents.

Besides cleaning small objects with a cleaning cloth or tar brush on the workbench, a component cleaner is widely used.

Immersion

 The products are placed in a bath with a cleaning agent for a certain time.The time required depends on the choice of cleaning agent and the possibilities that one has of movement and increasing the temperature.

The cleaning of metal products in immersion machines is a much-used method. This method is used for both solvents and for aqueous cleaning agents.

the positioning of the products in the bath is also of great importance. The products must be so positioned or separated from each other that they are sufficiently rinsed.

Spraying

High pressure cleaning

Vapour degreasing

Agitation methods with immersion

In combination with the immersion as a cleaning method, different ways of agitation (movement) are used, i.e.:- immersion rinsing or rotation - boiling-off - air agitation - liquid injection - ultrasonic cleaning.

Immersion rinsing or rotation

Boiling-off Air agitation

Liquid injection

Ultrasonic cleaning

Cleaning with alkaline agents

Depending on the material that must be cleaned, alkaline cleaning is subdivided into three categories, i.e.:- Strong alkaline, for steel and magnesium and heavy pollution - Weak to moderate alkaline, for light metals, copper, aluminium, zinc and reasonable pollution - Neutral, for sensitive metals and light pollution.

pH Composition Temp.

(C) Conditions for use

Strong alkalinepH 11 – 14

Surfactants Sodium (potassium) hydroxide Carbonates Silicates Phosphates Corrosion inhibitors Complexing agents

> 50 Steel  Heavy pollution

Weak alkaline pH 8 - 11

Surfactants Phosphates Silicates Borates Corrosion inhibitors Complexing agents

> 40 Light metals, copper, aluminium, zinc  Reasonable pollution

Neutral

pH 6 - 8

Surfactants > 20 Sensitive metals Low degree of pollution

Weak acid pH 3 - 6

Surfactants Organic acids Corrosion inhibitors

> 50 Steel Phosphorating Oxide

Strong acid PH 1 - 3

Surfactants Inorganic acids Corrosion inhibitors

ca. 20 Etching removal Rust removal

Safety

When degreasing with combustible solvents, we are involved with:

- danger of fire and explosion - toxicity of the solvents used.

In general, the vapour of the solvent starts to burn. A fire in a tray with solvent, above which the vapour has started to burn, can be put out by simply covering the tray. This is not the case if the liquid itself is also heated by a fire that has been burning for a long time and has even started to boil.

Fire and danger of explosion

If there is a lot of solvent vapour present in the air, then we get an explosive mixture that can be ignited by an electrical spark, tool sparks or friction.

Toxicity of solvents If you work in surroundings where the air can be polluted with harmful or toxic vapours, it is important to know how harmful these vapours are.

To express the toxicity and the danger of various substances in a number, the MAC value has been determined. MAC is the abbreviation of:

Maximum Acceptable Concentration.

With a low MAC value, the substance is therefore very toxic and with a higher MAC value less toxic. The MAC value is the concentration of the harmful substance that may be in the air during an eight-hour working day.

  MAC value Flash point

Methanol 200 ppm 11 C

Ethanol 1000 ppm 12 C

Acetone 750 ppm -19 C

Hexane 25 ppm -22 C

Benzene 10 ppm -11 C

1-methyl-2-Pyrrolidon (NMP)

100 ppm 96 C

Safety with alkaline cleaning is aimed at the chemical properties of these agents. Alkaline agents are very dangerous for the eyes, they cause immediate damage to the cornea.

Therefore, always wear special goggles that are completely closed or a facemask when working with, filling and mixing alkaline cleaning agents.

Concentrated alkalines soften the skin. Serious burn wounds are the consequence.

The more diluted bath liquids have a strong degreasing action on the skin, skin cuts. A strongly reduced resistance to infections is the consequence.

Therefore, always wear special, protective clothing and gloves when working with these agents.

If the (diluted) bath liquids come into contact with the skin, rinse thoroughly immediately and, if possible, neutralise the skin with a soap or soda solution. After drying the skin, apply a protective cream.