Technical

The Structure of thermal paper

Base Paper

The base paper used for most approved papers is a specially developed wood-free paper to meet the specific needs of thermal coating and only the highest quality papers can be used to ensure the desired quality characteristics can be achieved.

Undercoat /Pre-coat.

It is very important that an undercoat or pre-coat is applied to the base paper to ensure an even and smooth surface for the thermal coating to be applied. This practice ensures high resolution printing, image quality and prevents heat transfer into the paper so optimising the sensitivity of the thermal coating.

Thermal Coating

The thermal coating consists of three key elements including colour formers, colour developers and pigments. When thermal energy (heat) is applied a reaction or fusion takes place which results in the imaging of the paper generally in black.

Topcoat

An additional top coat can be applied over the thermal coating if the paper is likely to be exposed to specific external influences or requires special protection such as:

• Chemical influences through contact with oils, fats, varnishes or solvents.
• Being used within extreme environmental influences such as high humidity or temperatures.
• Physical abrasion such as scratching.
• The imaged paper requires an extended life.

The use of a top coat will also extend the service life of the print head.

Backcoat

A backcoat is strongly recommended when the reverse of the paper is exposed to external influences such as:

• Migrating adhesives
• Plasticizers such as PVC

The use of a backcoat will also improve the flatness of the paper under extreme climatic conditions.

Thermal paper sensitivity

The sensitivity of thermal paper refers to the degree to which it reacts to a specified amount of heat or energy. Paper sensitivity should be an important factor when selecting the correct thermal paper for your needs. It is usually shown in graph format which plot the development of image density (blackness) against the amount of heat or energy transferred.

Static Sensitivity

The static sensitivity indicates the temperature at which a thermal paper will begin imaging. Thermal papers with low static sensitivity only begin to image at high temperatures whilst papers with higher static sensitivity begin much earlier. The static sensitivity data is important if the thermal paper is being used in high temperature environments for example it is recommended that thermal papers with low static sensitivity are used for parking tickets where the temperature in a car can reach extremes during a hot summer’s day.

Dynamic Sensitivity

The dynamic sensitivity of thermal paper indicates how fast a thermal paper can be printed. This is particularly important when selecting papers for a specific printer performance as the higher the dynamic sensitivity the faster the printer can operate without any setting changes being required.

Storing Thermal paper

Avoid hot and humid environment

For long term storage keep thermal paper in the dark at an average ambient temperature of less than 25°C and a relative humidity of less than 65%.

Thermal papers begin to develop colour at between 60 to 100°C and reaches full image density at between 70 to 120°C. If the paper is used continuously in temperatures of 40°C or above for more than 24 hours attention should be given to the ambient temperature and humidity.

Avoid exposure to direct sunlight

Unused thermal paper will yellow and imaged papers will fade if exposed to direct sunlight for prolonged periods.

Imaged papers also tend to fade when left exposed to conventional fluorescent light for prolonged periods of time and as such important copies should be filed soon after printing.

Do not use solvent-type adhesives

Adhesives containing volatile solvents such as alcohols, esters, ketones, etc will cause colour formation.

Adhesives based on starch, PVA or CMC are harmless and as such recommended.

Avoid contact with plasticizers

PVC film contains plasticizers and prolonged contact reduces the image forming capabilities of the paper and can cause print image to fade. It is recommended that storage files made of polyolefin’s and polyesters are used.

Self adhesive cellophane tapes can contain plasticizer so if need arises to attach papers it is recommended this is done on the back side (uncoated side) of the paper.

Image stability guarantee

All image guarantees provided by Allrol Solutions Ltd apply only if papers are stored and handled under the above conditions and that any other specific issues detailed in any letter of guarantee are complied with.