Controlling Synthetic Polyester Dye Sublimation and Migration

Introduction

Since the first day when synthetic fabrics made their way into the screen printing industry, printers have been faced with the issue of dye sublimation and migration. Prior to the technological advancements made in dyestuffs, dyeing techniques and bleed resistant printing inks, dye sublimation and migration was a way of life. Still, with all the advancements made in textile printing inks, flash units and curing technology, dye migration and sublimation still plagues the textile screen printer. To fully understand the solutions to controlling the problem, one must first understand the characteristics of the components involved.

As with any aspect of textile screen printing, the variables involved in the scenarios are enormous. The aspects that must be taken into consideration when dealing with synthetic dye sublimation and migration are:

• Characteristics of synthetic dye sublimation.
• Characteristics of synthetic dye migration.
• Synthetic fabric content of the substrate.
• Synthetic dye type.
• Fabric dyeing and rinsing processes.
• Heat sensitivity of the fabric and dyes involved.
• Bleed resistance of the ink.
• Printing techniques.

The following information will review the aspects of each of these variables and the role each one plays in synthetic dye sublimation and migration.

Sublimation Defined

Synthetic dye sublimation is an interaction between the dyes employed in the coloration of synthetic fibers and heat. Regardless of the dyeing processes most if not all synthetic dyestuff will sublime (change from a solid state to a gaseous state) at a given temperature. The temperature at which a given dye will sublime is dependent on the types of synthetic dyestuffs, dyeing processes and synthetic fibers which were incorporated into a given substrate.

Migration Defined

Dye migration takes place as an interaction between the petroleum based polyester dyestuffs and the petroleum based plasticizer employed in plasti sol screen printing inks. The petroleum based plasticizer in plastisol inks devolve the residual petroleum based polyester dyes which allows the dyes to travel through the residual plasticizer network within the plastisol ink film until the dyes reach the surface of the ink film and appear as migration. Photo #1 shows an area of a print which the dye migration process has begun due to insufficient cure. The pink area within the ink film indicates synthetic dyes which are beginning to migrate into the ink film.

Synthetic Fabric Content Of The Substrate

In most cases, the possibility of polyester dye migration is directly proportional to the polyester content of the fabric. Garment manufacturers have employed polyester as a 100% fabric or as a blend for a number of advantages which polyester offers.

1. Polyester fabrics are resistant to shrinkage as opposed to cotton which can shrink as much as 5% or more. Incorporating medium to high polyester contents into a fabric diminishes the need to “over cut” a garment to allow for the mentioned shrinkage.
2. Polyester maintains a greater color strength via superior wash fastness and light fastness as opposed to cotton.
3. Polyester is relatively inexpensive as opposed to cotton and thus lowers the materials cost of manufacturing as well as the cost to the end user.

As the issue of synthetic dye migration becomes more and more apparent to garment manufacturers, manufacturers are offering garments (particularly fleece) with decreased polyester dye content. You can now find garments on the market with all combinations of polyester contents as opposed to the standard 50/50 cotton / polyester blend. A preferred blend for many textile printers is the 95/5 cotton – polyester blend fleece. A fabric of this type offers all the advantages of printing on 100% cotton while maintaining the shrinkage resistance of polyester.

Some manufacturers have also made progress in knitting processes wherein the fleece garment can contain a 50/50 blend of cotton and yet have the cotton portion of the fabric on the printing surface of the garment while the polyester portion is on the inside of the fabric. The advantage of a fabric such as this is that the ink will only be printed onto the cotton dyed portion of the fabric and be separated from the polyester… thus diminishing the probability of migration.

Synthetic Dye Types And Dyeing Processes

Although there are numerous dye types employed in the dyeing of synthetic fibers, dispersed dye are the primary dyes which are employed in the processing of most 50/50 tees and fleece products. Dispersed dyes, which are primarily employed for most polyester fabrics in the T-shirt industry. These dyes are set in to the polyester under strict conditions which allow the polyester fiber to swell during the dyeing process and absorb the dyes into the fiber. These dyes are normally dyed at 70°C (160°F) and pressurized during the dyeing process. Once these dyes are set into the fiber, they are resistant to thermal dye sublimation to 182°C (360°F). It is for this reason that many printers suffer synthetic dye migration when they heat the fibers beyond the 182°C point and release the (gaseous) dyes into the ink film. Once this occurs, the sublimated dyes can solidify within the ink film and begin the dye migration process.

Cationic dyestuffs are another dyestuff which holds particular interest for the textile screen printer in that when printing on fabrics which have been processed with cationic dyes, the fabric is quite resistant to dye migration. Cationic dyes require specially prepared polyester or acrylic fibers which allow themselves to be locked into the fiber during the thermosetting process. The down side to this option is that cationic polyester or acrylic will usually carry a higher price due to the required processing needed for the dyeing process.

Fabric Dyeing And Rinsing

From the standpoint of the fabric, there are two primary aspects of the fabric dyeing process which are important as to the degree which the fabric is likely to migrate or sublimate. First is the amount of dye being applied to the polyester fibers. During the dyeing of the polyester fibers, the fabric is heated to 82°C to 127°C (180°F to 260°F) at which point the polyester dye sites expand and allow for the absorption of the dyes. Once the polyester fibers have been dyed, residual dyes can be left on the surface of the fabric. It is these dyes which are susceptible to dye migration.

The second is the rinsing process is employed to remove those residual dyes from the surface of the fabric. Should a fabric have a sufficient dye content and insufficient rinse process, the fabric will have a greater probably of migrating. Another aspect to consider is the weight of the fabric. A 6 oz. 50/50 fleece will not contain the dye content of a 9 oz. 50/50 fleece since there is obviously less polyester by weight. A heavier weight fabric will require a greater amount of dye than a lighter weight and thus will require a greater amount of rinse during processing to remove the residual dyes.

Heat Sensitivity Of The Fabrics/Dyes

Although the dyes in most commercial polyesters are heat set to retain excellent color fastness, most polyester dyes used in T-shirts and fleece can withstand temperatures up to 182°C (360°F). It is at this point that the polyesters sublime into a gaseous state and penetrate the plastisol ink film. In most quality dyed fabrics, it is the residual polyester dyes on the surface of the fabric which sublimate and not the dyes within the fibers.

The value to knowing this fact is understanding the reaction between the dyes in the polyester fabric and the heating process during flashing and curing. Most dye sublimation takes place during the flashing while the fabric is exposed to 482°C to 538°C (900°F to 1000°F) from the flash unit. It is this fact that dictates that printers need to print at slower speeds with the flash unit temperature decreased as opposed to super-heating the ink and garment in the name of productivity.

Bleed Resistant Plastisols

As the textile screen printing industry has evolved to higher quality printing through high tension, thinner ink films and softer prints, the ink manufacturers have been charged with improving bleed resistance of plastisols to deter dye migration while printing under these conditions. The products employed today range from bleaches to organic peroxides. As OSHA and the EPA have invoked stricter guidelines on the manufacturing of plastisols products, many manufacturers have moved away from bleaches to organic peroxides. Plastisol manufacturers also employ fillers which increases opacity and assist the dye blockers in dye migration.

The degree to which a plastisol will deter synthetic dye migration is dependent on the type and amount of dye blocking agent employed in the inks formulation. As opposed to the offset printing industry which employs SWOP (Standard Web Offset Printing) inks the textile printing industry has no standards to which the plastisol manufacturers can go by.

This is partially due to the wide range of substrates on which a plastisol must perform. Due to this fact it is up to the textile printer to determine which product is to perform best on the fabrics which he/she prints. Since no two batches of plastisol or two lots of polyester dyed fabrics are exactly the same, it pays to print-test random lots of inks and fabrics on a regular basis to ensure that the products are going to perform up to required quality standards.

The level of cure which is bleed resistant is also crucial in deterring dye migration. As a plastisol cures, the PVC resin in the plastisol swells under the heat and absorbs the liquid plasticizer to solidify the ink film. Should all of the available plasticizer not be absorbed the residual plasticizer will solvate the available dyes on the fabric and allow the dyes to migrate through the available plasticizer network until the dyes reach the surface of the ink film. This can take place within 2 hours to 6 weeks. Due to this fact, if a plastisol requires 160°C (320°F) to properly cure, it is crucial that the entire ink film thickness reach that temperature to properly cure and deter dye migration.

Printing Techniques And Fusion Parameters

Knowing the dye migration is caused by a solvating interaction between the plasticizers in the plastisol and the residual polyester dyes on the fabric, the logical solution is to minimize the contact between the ink and the fabric. High tension printing allows the ink film to be deposited on the surface of the fabric as opposed to pressing the ink into the fabric from the excessive squeegee pressure required of low tension screens.

Photo #2 shows a cross section of the same fabric with the ink deposited into the fabric under low tension printing conditions and the ink deposited onto the fabric under high tension printing condition.

Having the ink pressed into the fabric allows the garment to act as an insulator for the ink and will thus require additional time and temperature to achieve a proper cure. Retensionable printing frames should be employed when printing on synthetic or combination fabrics to allow for a minimizing of the ink/fabric interface. In addition to minimizing the ink/fabric interface, high tension screens will deposit a thinner and more opaque ink film onto the surface of the fabric. The thinner ink film will require less time and temperature during flashing and curing process; in turn minimizing the probability of thermal dye sublimation.

From the standpoint of dye sublimation within the printing process, the key to controlling the process is to minimize the amount of heat which the fabric and ink are exposed to during the flashing process. Although many printers feel that their fabric temperature does not reach 182°C (360°F) while flashing, please consider the surface temperature of the flash panel and the proximity of the fabric to that panel.

Should the flash panel be running at 482°C to 538°C, the amount of time it will take for the polyester to reach 182°C will be scant seconds. Although the garment remains under the flash unit for only a matter of seconds, the difference in dye stability and sublimation can be as little as a half a second. It is advantageous to decrease the heat of the flash unit and slightly decrease the cycle time during the flashing process if you suspect sublimation as a contributing factor in any migration issues which you might be experiencing.

It starts to become apparent at this point that the thermal window which textile screen printers must work within is quite small. Since most plastisol screen printing inks require 160°C (320°F) of heat to properly cure and while keeping the polyester fibers from reaching 182°C (360°F), great care must be taken in the amount of heat which the fabric and ink are exposed to, not only in the flashing process but also in the curing process. Of the available curing units available today, forced - air gas dryers offer the greatest amount of heat control as opposed to conventional radiant heat dryers. Forced air - gas dryers rapidly raise the ink film temperature to 160°C and plateaus for a given period of time as opposed to radiant.

Standard radiant heat dryers will slowly heat the ink until the ink film reaches 320 degrees. In most cases, this point is reached just prior to the garment exiting the dryer. Although there are radiant dryers which can plateau around the 160°C (320°F) mark, the plateau is erratic since the many radiant dryers work off of thermostats or percentage timers.

Conclusion

Synthetic/thermal dye sublimation and migration are manageable issues as long as there is a clear understanding of the interactions between the synthetic dyes, inks and heat exposure. Once you have identified the potential for migration issues within a given fabric, the appropriate safeguards and procedures can be put in place to address the issue.