color matching control

The appearance of a product has a great deal of influence on the consumer's decision to purchase. A consumer or professional buyer will expect the article and its different components to be homogeneous in their overall appearance. Colour management is a delicate process and requires reliable tools and constants in order to meet the quality levels demanded. In a strong position due to their expertise in vision technology, CTC studied the best performing colorimetric measurement systems for leather and have developed a mechanism/device to automate the batching of pieces of leather for leathergoods.


The pitfalls of colour in leathergoods

Colour is very difficult to pin down: what we see is not always what we think we see; however, what we think we see can be measured. Also, during the manufacture of leather products such as wallets, the management of pieces of the same colour is a delicate stage.

Currently, production workers carry out a visual control of the pieces and samples coming from the cut: the various parts of the neck and body for, say, credit card holders, taking the different substances into consideration.

They ensure that perceived colours match, in order to reassemble the pieces by batch. Over a hundred pieces can be treated at the same time. One must recognise the colour differences between them, ensuring that:


pieces match from one skin to another
pieces from a single skin can be used together
Colour matching in leathergoods is an essential procedure. If pieces have not been matched exactly, a repair may be necessary after assembling the article. This would involve searching the stock for a piece in the appropriate colour.

Furthermore, it can be interesting to control the colour of pieces made in different materials (leather, textile etc) so that once assembled, a multi-material product appears uniform in colour.

Limitations of current controls

This operation is currently carried out manually and by the same person throughout the working day. This control has several limitations:


perception of colour varies between individuals
the human being has poor colour memory
the difficulty of communication on the subject of colour
colour varies according to the light
perception of colour deteriorates with fatigue
Objectives of the project

The project, led by CTC, aimed to validate a method of colorimetric analysis, and then an automatic tool capable of matching pieces of leather or other material, which are to be assembled into one single product.

This system is capable of analysing colour and brilliance of different materials by generating a ‘harlequin' effect1, as well as controlling the presence of metamerism2. This tool is objective, fast, precise and constantly reliable.

Optimal batching for leathergoods

In order to correctly analyse the colour of a leather, CTC opted for a spectrocolorimeter equipped with a sphere of diffusion capable of setting itself free of the texture of the leather and only extracting the information which pertains to the characteristics of colour, which allows the control of different types of materials in a single batch.

The data supplied by the spectrocolorimeter (reflection curve) are not usable in this state to characterise the perceived colour of the sample, so CTC decided to develop two systems which meet differing objectives and costs. A choice can be made between a validation of manual sorting and an automatic batching.

Validation of manual batching: control via CMC sorting

This control is carried out with the help of a portable spectrometer with an integrated sorting programme. This control is traced through the provision of an existing visual control which the operator has introduced a priori to the control. The operator chooses a sample from among a ‘population' which will be referenced as ‘standard' from the batch of pieces which will make up a single product.

Automatically, the system will define an ellipse of acceptability around a point measured in the 3D HSL space (HSL = hue, saturation, luminosity). This space allows us to more easily describe colour using intelligible data.

This ellipse is measureable according to the colorimetric distance tolerated by the client and standardised by the International Commission on Illumination (CIE).

Furthermore, it adapts automatically to the analysed colour as human vision is more sensitive to green than black for example.

The ellipse will be narrower around the point in the green and the tolerance will be more important in the black. Straight away, the operator analyses a second sample which he deems suitable for the batch, and the system indicates whether the sample is in fact acceptable, ie that it is located within the ellipse of acceptability: if so the system displays ‘Good'. If not, it indicates ‘Poor'. The operator repeats this process until a complete batch is obtained.

Furthermore, this method allows us to resolve the problem of a potential repair after the product has been assembled. Even if it is just one piece that has to be replaced, the operator can simply analyse a neighbouring piece from the product and analyse a replacement piece or a skin from the stock which falls within the defined colorimetric distance. Due to the system's portability, this stage is easy to carry out.

Automatic control

In this method, the operator will measure the set of parts of the same colour and the apparatus will automatically sort batches of parts according to the colour parameter. Thus the operator will be able to assemble parts from the same batch without worrying about the colour. The visual matching should be carried out only according to the grain and the brilliance.

a. Optimal method

After having been ‘flashed' with the spectrocolorimetry, all the data on the samples desired are transferred to a computer. The treatment is carried out with a colorimetric software programme developed by CTC, allowing batching of samples according to the number of samples per batch, the light, the colorimetric colour distance parameters tolerated by the client and standardised by the CIE with a management of the harlequins.

This method is awkward because it requires handling samples without automating the station. CTC opted instead for the sorting 555 method which allows the user to obtain a code for each sample after each measurement.

b. Sorting 555

Sorting 555 is a partition of colour distance in a box whose dimensions correspond closely to the colours tolerated by the company. All the samples present in this box are accepted and receive the same colour code.

This method is well suited to the challenges of automatic batching, because the sample's classification code is obtained directly after measurement and thus the colour category of the sample is directly affected. Moreover, by using this method to finish batches we carry out regroupings of boxes, and have a perfect command of the error induced according to the components of bringing together L and a or b.

Promising results

This system also allows us to take into consideration the phenomenon of metamerism which is present on certain leathers, according to the acceptable colour distance under the primary source of light defined by the user (for example D65 ‘daylight'). The system also calculates the colour distance between two samples under a secondary light source (eg F11 flourescent tube') and defines the metamerism as the colour distance under the secondary light source, having restored the colour distance under the primary light source to zero.

The user specifies a variation value of the colour distance between the two defined light sources. A batch may be acceptable under one light source but rejected if the notion of metamerism is taken into account.

The system perfected by CTC allows them to improve the reliability (unchanging to visual fatigue, calibrated daily against a reference slide and integrating all types of lighting into one measure) and the productivity of batching in leathergoods (less than one second for the analysis of a piece).

Understanding colour

A small theoretical detour is essential to understanding what is at stake in the control of colour.

In a human being, the notion of colour is intuitive: it's a phenomenon which is interpreted by the brain of the observer, of an image perceived by the eye. Hence the subjectivity of judgements and the difficulties in communicating about colour. In perceiving a colour, one must analyse three parameters:


The light source
The angle of light source and observer
The object
The observer/receiver
The light source causes colour to vary considerably: fluorescent light in production, daylight, shop lighting...

The object absorbs, diffuses, reflects, diffracts and refracts the light. According to its texture, its relief, its colour and its capacity to reflect, it will resend a spectre of light to the viewer. The viewer or observer picks up this light: it must be calibrated according to the source of light in the scene being observed. And in this case, the calibration of the measuring apparatus will always be easier than that of the human cortex...

Colour is deceptive: subject to interpretation, it varies constantly.

Furthermore, colorimetry provides elements for analysis with the goal of specifying colour, from the starting point of its three constituent parts.

Footnotes

1 The management of harlequins consists in establishing an order of positioning of the samples in a batch according to the increasing colour distance

2 With metamerism, two samples of which the colorimetric distance is acceptable under a given light source (natural light for example) and is not greater under another light source such as a fluorescent tube

Restricted formalin harmful chemical

Formaldehyde is a colorless gas bp -21 degree c. It has an irritating odour. It is extremely soluble in water. Formaldehyde is sold as 40 % aqueous solution under the name FORMALIN and is used in this form for most purposes. Formalin is used as a jeneral aniseptic. In leather industry for good fastness (physical fastness ) property.

Formaldehyde is a chemical compound (also known as methanal) that is widely used in many industries. It is the simplest aldehyde chemically with the formula H2CO. Formaldehyde is a gas at room temperature but it is also readily soluble in water (and is often sold as an aqueous solution).


Areas of application include adhesives, textile processing, as a preservative in some paints, coating products and also in some cosmetics. Formaldehyde is also used in the production of some polymers. When combined with phenol, urea or melamine, formaldehyde produces a hard thermoset resin. It is also used during embalming processes to preserve corpses.

Potential exposure routes for formaldehyde are as wide ranging as smog, cigarettes and tobacco smoke and some household sources such as fibreglass, carpets, foam in cushions, permanent press fabrics, paper products, household cleaners, shampoos, bubble bath, medicines and disinfectants.

Restricted?Why is it

The use of formaldehyde in many construction materials means that it is a common indoor air pollutant. At concentrations above 0.1ppm in air, inhaled formaldehyde can irritate the eyes and mucous membranes, potentially resulting in watery eyes, headache, a burning sensation in the throat, and difficulty breathing. Formaldehyde is also classified as a carcinogen.

How is it relevant to leather?

Historically, formaldehyde has been used as a tanning agent due to its ability to crosslink proteins such as collagen. In addition, it has been used (again historically) to crosslink protein finishes such as casein.

Formaldehyde or materials that are formaldehyde release agents may, however, be used in some areas of tanning, specifically in the production of wet-white, to allow stabilisation prior to splitting or shaving and in the production of syntans.

In modern processing, formaldehyde can be used in the manufacture of certain polymeric-based synthetic tanning agents. Its presence in leather may be as a result of the condensation residue from some synthetic tanning agents.

Theoretically, formaldehyde should be fixed to the collagen during processing (considering that formalin is used as a preservative because of its reactivity with protein). However, some of the reactions used in the preparation of syntans are reversible (eg production of melamine formaldehyde resins). This means that, under certain conditions, it is possible that formaldehyde is liberated. Also oxidisation of oils/fats can result in formaldehyde formation.

Practical advice on avoiding formaldehyde formation (as determined by the current industry test methods) includes:


Careful management of formaldehyde releasing compounds, for example:
- Aldehyde tanning agents

- Formaldehyde resin retannages

- Preservatives (formaldehyde can sometimes be used as a preservative in processing chemicals)


The use of oxidisable oils should be carefully controlled as these can oxidise on exposure to air, moisture and heat, resulting in possible formaldehyde formation.
Adding reducing agents to the float (such as sodium metabisulfite) may help avoid formaldehyde liberation.
What is the legislation?

Restricted substance legislation is highly variable depending upon the final application of the leather in the product and the target user. Also there are considerable variations in legislation depending on the country where the leather or product is manufactured or sold. It is one of the key examples of how variable the restrictions can be.

Within Europe there is no general legislation that limits the presence of formaldehyde in leather. There are some individual countries that have restrictions on its presence in consumer products and formaldehyde is restricted within the recently published EN71-9 Toy standard. Various Eco-labels require that levels of formaldehyde are determined and the automotive industry tends to have quite strict limits on formaldehyde release from car interior materials. Table 1 lists some of the limits currently in place for formaldehyde. In addition to these restrictions on the extractable levels of formaldehyde present in leather, there are also some Eco-labels (Blue Angel for example) that have restrictions on the volatile formaldehyde released from leather and products.

BLC guidelines state that leathers should contain no more than 200ppm of formaldehyde for articles in general use. If the item is in direct skin contact this should be 75ppm, and 20ppm for items used by babies (<36>Methods of analysis

There are three separate methods used in the leather industry for the analysis of formaldehyde. Their application depends on the final use of the product along with the technical level of the laboratory carrying out the analysis. The most commonly used methods for analysis are described in CEN ISO TS 17226. This has two parts: a colorimetric method and an HPLC method.

Colorimetric method: This involves a colorimetric determination of the extractable formaldehyde. The leather is extracted at 40°C, after which the extract is treated with acetyl acetone. A yellow compound (3,5-diacetyl-1,4-dihydrolutin) is formed in the presence of formaldehyde, which is quantified photometrically at 412nm. Within this method there is a check to determine the presence of other compounds that may result in a coloured compound when reacted with acetylacetone. This involves addition of Dimedone to the extract prior to addition of the acetylacetone. If the resulting solution has an absorbance in excess of 0.025 (for a 1cm cell) there is the possibility of a false positive result being obtained.

HPLC method: This is based on determining the extractable formaldehyde using high performance liquid chromatography (HPLC). The formaldehyde is extracted at 40°C, after which the extract is reacted with 2,4-dinitrophenylhydrazine (DNPH). The resulting extract is then separated using HPLC, with UV detection at 350nm.

The CEN and ISO committees responsible for these methods are currently in the process of converting them to become full standards. When complete they will be referenced as EN ISO 17226-1 and EN ISO 17226-2. Part 1 is the HPLC method and will include a statement to the effect that this is the more reliable technique to be used in any case of dispute. This is due to the potential for interference and false positive results that may occur with a colorimetric analysis.

Also used in the leather industry but to a lesser extent is the Japanese method JIS L 1041 - 1983 (revised 1994). This is similar to the colorimetric method listed in CEN ISO TS 17226, with some modifications to the extraction procedure.

In addition, the automotive industry uses a separate analysis based on a headspace extraction technique. Within this method the sample is suspended in a sealed container over a defined amount of water. After heating, any formaldehyde liberated by the leather should be dissolved in the water. This resulting solution is then analysed colorimetrically. Acceptable limits as analysed by this method are typically 10ppm.

Conclusions

Testing of formaldehyde in consumer products is not straightforward. Even within a single material, such as leather, there are several testing methods that can be applied. Different materials (such as wood, cosmetics and textiles) will also have their own test methods that are appropriate. In addition to the complexity of the testing, there is also the variability in legislation and regulations that affect the products. BLC is able to offer support and advice to customers who have concerns about the presence of formaldehyde in their products

sweaty foot fill gap footwear comfort

one of the most important requirment of leather footwear is its resistance to sweatif it is sweatless it become comfortable

As part of its comprehensive comfort testing programme (which includes advanced moisture management test AMMT, and whole shoe thermal insulation test WSTIT), Satra Technology Centre, located in the UK, have designed and created a new small size 'sweating foot' component to meet growing demand to test women's and children's footwear for breathability and comfort.



The new moulded foot is based on a UK size four foot and considerably enhances the testing facility. Since introducing the comfort testing programme, it has been used continually to help manufacturers across the world develop footwear that remains dry and performs at a comfortable temperature.

By using a heated and 'breathing' foot form to simulate a realistic in-shoe environment, AMMT effectively creates a picture of how much sweat has been absorbed into each part of the footwear system and how much has permeated to the atmosphere. Based on the results, a manufacturer is better placed to make a decision on whether or not to make adjustments to footwear design and the materials used in production.

Satra's comfort testing equipment has been using foot forms in UK sizes seven and nine to arrive at findings to cover most of the adult market; the new size four foot will allow clearer insight for product aimed at the children's and part of the women's market.

Data from testing can be used to monitor and improve sweat management and thermal properties in all types of footwear including sports, performance, outdoor and industrial, as well as everyday shoes. Satra also uses its own climate chambers for the testing of performance and comfort properties of footwear.

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