Plasticizer migration is a problem whose importance can easily be underestimated when designing butyl tapes and sealing cords. Immediately after production, a compound may meet every quality-control requirement, exhibiting the required plasticity, surface tack, penetration and adhesion. Undesirable changes may only become apparent after weeks or months in storage, or during service when the seal is exposed to temperature, pressure and contact with a substrate.

An oily film may then appear on the tape surface, a dark halo may develop around the butyl cord, the release liner may become translucent, and concrete or stone may become discolored. Sometimes adhesion initially increases, only to decline markedly later. The problem is often summed up as “the plasticizer is migrating.” From a processing and formulation standpoint, however, this is an oversimplification.

Similar symptoms may arise from different processes: diffusion of low-molecular-weight components, phase separation, oil absorption by the substrate, evaporation of light fractions, liquid being squeezed out under pressure, or even creep of the entire uncrosslinked compound. Correctly identifying the mechanism is essential to avoid improving one formulation parameter at the expense of others.

What exactly is plasticizer migration?

A plasticizer or another low-molecular-weight liquid component is generally not chemically bonded to the elastomer chains. It occupies the space between macromolecules, increases their mobility and thereby reduces the viscosity and modulus of the material.

From a mass-transfer perspective, migration primarily involves movement of a component from the interior of the compound toward its surface. It may then remain on the surface, evaporate or transfer into an adjacent material. Diffusion is described by Fick’s laws. The rate of transport depends, among other things, on the diffusion coefficient, temperature, concentration and molecular weight of the plasticizer, and mobility of the polymer matrix.

In practice, four phenomena that can produce similar symptoms must be distinguished.

1. Internal migration

A liquid component moves from inside the compound toward the surface. This is the classic diffusive transport mechanism.

2. Liquid-phase exudation (bleed-out)

Droplets or an oily film appear on the material surface. Bleed-out can indicate not only rapid transport of a component but also that its solubility limit in the polymer system has been exceeded. In this situation, the compound is no longer thermodynamically single-phase.

Studies of butyl rubber (IIR) systems indicate that limited oil solubility can lead to a multiphase structure consisting of a swollen rubber matrix and dispersed oil-rich domains. A compound may therefore appear homogeneous immediately after manufacture even though its structure favors subsequent liquid bleed-out.

3. Contact migration into the substrate

The plasticizer does not always accumulate on the butyl surface. If a material in contact with the tape has a strong affinity for the liquid phase, it can gradually absorb that phase and act as a sink for the migrating component. This particularly concerns:

• porous concrete and natural stone;
• bituminous materials;
• certain paint and coating systems;
• other elastomers;
• some plastics, including those containing plasticizers of their own.

As a result, an oily halo may appear on the substrate even when no distinct oil layer is visible on the tape itself. ASTM C772, which addresses oil migration and plasticizer exudation from preformed sealing tapes, provides a reference for evaluating this phenomenon. In certain applications, slight, controlled migration can assist penetration into a porous substrate. Nevertheless, uncontrolled migration generally compromises seal quality and durability.

4. Cold flow mistaken for migration

An uncrosslinked butyl compound is viscoelastic and can creep under sustained loading. Material flowing beyond the joint therefore does not necessarily indicate migration of the plasticizer alone. The entire compound may move, including the polymer, filler and liquid phase.

This distinction has immediate practical implications. Choosing a more compatible oil may reduce bleed-out without necessarily improving creep resistance. Conversely, increasing the degree of crosslinking may effectively limit cold flow but will not resolve poor plasticizer compatibility.

Why are butyl tapes and cords particularly susceptible to migration?

The service conditions of these products favor movement of liquid components. Butyl tapes and sealing cords typically feature:

• a high surface-area-to-volume ratio;
• direct contact with one or more other materials;
• a relatively high liquid-phase content;
• the need to operate over a wide temperature range;
• the need to retain high tack and substrate wet-out;
• simultaneous requirements for dimensional stability and resistance to extrusion from the joint.

Reducing plasticizer content can be relatively straightforward in a conventional rubber compound. In butyl tapes, liquid ingredients such as oil, liquid polyisobutylene (PIB) and polybutene simultaneously influence processability, extrusion, surface tack, substrate wet-out, compressibility and low-temperature flexibility.

Formulation design therefore requires a balance between the mobility needed for intimate, leak-tight contact with the substrate and the stability required to keep the seal in the joint.

Which ingredients act as plasticizers in a butyl compound?

For butyl compounds, the term “plasticizer” is sometimes used too broadly. A tape or cord formulation may contain:

• butyl rubber (IIR);
• medium- or high-molecular-weight polyisobutylene (PIB);
• liquid PIB or polybutene;
• mineral oil;
• tackifying resins;
• mineral fillers and pigments;
• processing aids;
• a crosslinking system.

Liquid PIB or polybutene may simultaneously function as a plasticizer, viscosity modifier and tack-promoting ingredient. A blanket recommendation to “reduce the plasticizer content” is therefore insufficient. First, the fraction that is actually migrating must be identified: mineral oil, low-molecular-weight polybutene, a resin constituent or a combination of substances. Only then can the formulation be rationally adjusted.

Why is compatibility more important than plasticizer concentration alone?

Butyl rubber is hydrocarbon-based and relatively nonpolar. It can therefore be compatible with suitably selected paraffinic and naphthenic oils, as well as polybutenes. Formulation literature links this compatibility, among other factors, to similarities in the solubility parameters of the individual ingredients.

This does not mean that every paraffinic oil will perform equally well. Relevant factors include:

• molecular-weight distribution and viscosity;
• the proportion of light fractions and degree of refining;
• paraffinic or naphthenic character;
• interactions with PIB and the tackifying resin;
• filler type, loading and specific surface area.

Compatibility between IIR and the oil does not guarantee compatibility among all formulation ingredients. This is a frequent source of incorrect assessments during compound development.

Why does a plasticizer start to migrate? Seven main causes

1. Exceeding the matrix’s capacity to retain the liquid phase

Every system has defined limits of thermodynamic compatibility. As oil content increases, the oil may initially remain dissolved in the polymer phase or be very finely dispersed in it. Above a certain concentration, however, a separate liquid phase may form.

Macroscopic separation need not be visible. Oil-rich microdomains may develop within the material, from which the component gradually migrates to the surface. Thus, a compound containing 20 phr of a well-matched oil may be more stable than one containing 15 phr of an incompatible oil. The phr value—parts by weight of an ingredient per 100 parts by weight of rubber—does not, by itself, determine migration resistance.

2. Plasticizer molecular weight is too low

Smaller molecules generally move more easily between polymer segments. Increasing the molecular weight of the plasticizing component or using suitable oligomers can therefore reduce its diffusion. In butyl compounds, this supports using higher-molecular-weight liquid PIB or polybutene grades.

This approach involves trade-offs, however. It usually increases compound viscosity and makes extrusion more difficult; in some cases it also impairs substrate wet-out and low-temperature tack. There is no universally optimal polybutene grade: the choice depends on processing requirements and the product’s service conditions.

3. Excessively high service temperature

Rising temperature increases the mobility of polymer segments, accelerates diffusion of liquid ingredients, lowers compound viscosity and may intensify evaporation of light fractions. A tape that performs correctly at 23 °C may therefore undergo noticeable changes after several weeks of exposure at 70 °C.

For this reason, evaluating migration only at room temperature is insufficient for products intended for roofing, automotive or industrial applications.

4. Excessive total liquid-phase content

Assessing oil content independently of the other liquid ingredients is a mistake. The balance should include oil, liquid PIB or polybutene, liquid fractions of tackifying resins and other low-molecular-weight additives.

Individual ingredients may be compatible with IIR, but their combined loading may exceed the ability of the overall system to maintain a stable liquid phase.

5. Insufficient high-molecular-weight polymer content

High-molecular-weight IIR and PIB chains form physical entanglements that hinder compound flow and can restrict transport of smaller molecules. Increasing the effective polymer-phase fraction is therefore one way to improve stability.

Earlier butyl-tape formulations used, among other approaches, a higher polymer fraction and partially crosslinked IIR to improve material strength while retaining ease of application.

6. Incorrect filler selection or dispersion

Fillers do more than increase product weight and reduce formulation cost. They can significantly affect the distribution of the liquid phase. Important parameters include specific surface area, particle size and shape, surface chemistry and treatment, moisture content and degree of dispersion.

A high-specific-surface-area filler may partly restrict liquid-phase mobility. If it is incompatible with the hydrocarbon matrix or insufficiently dispersed, however, a heterogeneous microstructure results. Areas rich in polymer, filler or oil may develop within an apparently homogeneous compound, promoting later migration.

7. Incorrect mixing sequence

Adding oil too early can sharply reduce batch viscosity and the shear stresses needed to disperse fillers. The compound then becomes easier to process, but its microstructure may be less homogeneous.

For this reason, in rubber compounding the plasticizing ingredient is often metered in only after part of the solid phase has been adequately dispersed. When producing butyl tapes and cords, the order in which raw materials are added should be treated as an important processing parameter, not merely a matter of mixing logistics.

Uncrosslinked versus partially crosslinked compounds – what is the difference?

Standard uncrosslinked butyl compound

Uncrosslinked butyl tapes and cords retain pronounced viscoelastic behavior. They wet substrates well, conform to irregularities, exhibit high tack, readily fill gaps and can remain plastic for extended periods. Under favorable conditions, the material can also be re-extruded.

These same properties impose limitations: greater susceptibility to creep, cold flow, squeeze-out beyond the joint, movement of the liquid phase and temperature-induced property changes. Uncrosslinked butyl is therefore not an inferior version of a crosslinked material; it is simply suited to different service conditions.

What does crosslinking change?

During crosslinking, bonds form between polymer chains, creating a three-dimensional network. These bonds restrict permanent movement of chains relative to one another. Depending on composition and degree of crosslinking, they can improve:

• resistance to creep and squeeze-out;
• dimensional stability;
• performance under sustained loading;
• resistance to deformation at elevated temperatures.

Reduced polymer-segment mobility may also slow the diffusion of small molecules. Partially crosslinked butyl is therefore used where sealant properties must be retained while improving resistance to deformation. The extrusion of thin tapes from dynamically, partially crosslinked IIR compounds plasticized with PIB, with improved mechanical strength, has also been described.

Crosslinking does not chemically bind the plasticizer

If mineral oil does not participate in the crosslinking reaction, formation of bonds between rubber chains does not chemically bind it to the matrix. The oil remains mobile. The network may restrict its mobility, impede transport and reduce compound deformation, but it does not eliminate thermodynamic incompatibility between plasticizer and polymer.

Can excessive crosslinking increase oil bleed-out?

Under certain conditions, yes. If the compound is already near the liquid phase’s solubility limit before crosslinking, forming a stiffer network changes polymer mobility, swelling equilibrium and the system’s local structure. As a result, some liquid may be expelled from the polymer phase and separate more readily at the surface.

The rule “more crosslinking means less migration” is therefore not universally valid. Properly designed partial crosslinking of a compatible compound may reduce liquid-phase mobility and creep, but it cannot compensate for an improperly balanced formulation.

Standard versus partially crosslinked butyl – property comparison

The key advantage of crosslinking is not an automatic increase in adhesion but better control of cohesion and rheological properties over time. Earlier patents concerning butyl tapes already indicated that combining crosslinked and uncrosslinked IIR could increase material strength while maintaining appropriate flexibility and processability.

Why is completely eliminating migration not always the right objective?

Butyl tape must effectively wet the substrate and conform to its microscopic irregularities. This requires a sufficiently low surface modulus and the ability to deform and flow locally.

Drastically reducing plasticizer content, increasing the fraction of high-molecular-weight PIB, over-crosslinking IIR or adding large quantities of active filler may reduce migration. At the same time, however, they can impair tack, substrate wet-out, low-temperature ease of application and the ability to fill irregularities. They may also increase the force required to compress the seal.

The objective should therefore not be maximum oil retention at any cost. Maintaining controlled component mobility while preserving long-term cohesion and seal functionality is more important.

How can plasticizer migration in butyl tapes and cords be reduced?

1. Improve ingredient compatibility first

Before increasing filler loading or changing the degree of crosslinking, assess the mutual compatibility of IIR, PIB, plasticizer and tackifying resin. If the system is phase-unstable, additional ingredients may only make the problem less visible without removing its cause.

2. Select the molecular weight of the plasticizing phase

Some of the low-molecular-weight liquid phase can be replaced with higher-molecular-weight liquid PIB, more viscous polybutene or a suitably compatible oligomer. Such ingredients generally diffuse more slowly. Care must be taken, however, not to increase the overall compound viscosity excessively. Combining two or more fractions of differing molecular weight may be beneficial.

3. Increase the effective polymer-phase content

Depending on the formulation, options include IIR with higher Mooney viscosity, high-molecular-weight PIB, partially crosslinked IIR or a blend of several compatible polymers. The aim is not simply to increase hardness, but to build a structure that retains the liquid phase more effectively and provides adequate cohesion.

4. Consider controlled partial crosslinking

For many tapes and cords, partial crosslinking may offer the best compromise. The material should still adhere to the substrate, deform under pressure, fill irregularities and remain suitable for extrusion or calendering. At the same time, the polymer network should limit creep, squeeze-out and movement of the entire polymer phase. The highest possible crosslink density does not necessarily deliver the best performance.

5. Select the tackifying resin correctly

A tackifying resin is not an independent adhesion control but part of the overall system. Changing it can affect the glass transition temperature of the continuous phase, viscosity, IIR–PIB compatibility, oil distribution, tack and cohesion.

Replacing one resin with another at a 1:1 ratio, even at a similar softening point, may therefore substantially alter the compound’s tendency to migrate. Softening point alone does not measure compatibility.

6. Optimize the filler system for rheology

When selecting calcium carbonate (CaCO₃), talc or silica, analyze not only their formulation loading but also particle size distribution (PSD), specific surface area (e.g., measured by BET), morphology, surface treatment, oil absorption and moisture content.

Two CaCO₃ grades with similar mean particle sizes may differ considerably in their effects on rheology and compound stability.

7. Control the mixing process

A temperature that is too low can hinder ingredient dispersion, whereas excessive temperature may overly reduce viscosity, promote loss of light fractions, change resin properties and initiate uncontrolled reactions in the crosslinking system.

For partially crosslinked compounds, monitor not only the final temperature but also the material’s residence time at individual process stages and its complete temperature profile.

How can you identify what is actually migrating? Diagnostic methods

Visual inspection is a useful initial indicator but is insufficient to establish the cause. Effective diagnosis should combine several complementary tests.

Test 1. Mass balance

Store a specimen in contact with a controlled absorbent material and, after aging, measure the mass change of both the butyl compound and the contact material. This helps distinguish actual component transfer from a change in seal shape alone.

Test 2. Testing in accordance with ASTM C772

ASTM C772-03(2025) addresses the evaluation of oil migration and plasticizer exudation from preformed sealing tapes. It can therefore serve as a useful reference for butyl products. A single test result should not, however, replace complete product validation under the intended service conditions.

Test 3. FTIR or GC–MS analysis of the migrating fraction

If an oily halo appears on the release liner or substrate, its chemical composition should be determined. Analysis can help establish whether mineral oil, polybutene, a resin constituent or another additive is migrating. Without this identification, formulation changes largely amount to trial and error.

Test 4. Contact-aging tests on actual substrates

Testing on glass or stainless steel alone does not represent every service condition. If the seal will contact PVC, painted aluminum, concrete, EPDM, polyethylene (PE), a roofing membrane or a powder coating, those materials should be included in the test program.

The tendency to migrate depends not only on the butyl compound but also on the properties of the material with which it is in contact.

Test 5. Rheological and creep-resistance testing

Determine whether observed extrusion is caused by migration of the liquid phase or creep of the entire compound. Rheological measurements and creep or sustained shear-load (static shear) tests are useful. Without them, the two mechanisms are easily confused.

Test 6. Gel fraction and swelling of crosslinked compounds

Simply calling a material “crosslinked” says little about its structure. It is useful to assess gel fraction, swelling ratio, crosslink density and crosslinking uniformity. Two products marketed as “crosslinked butyl” may differ fundamentally in structure and in-service behavior.

“Crosslinked butyl” – what does the term actually mean?

On the market, “crosslinked butyl compound” may refer to several different technologies:

• Variant A – addition of partially crosslinked IIR. The remainder of the compound retains a thermoplastic or plastic character.
• Variant B – crosslinking of the entire rubber phase during manufacture. A more continuous network develops.
• Variant C – dynamic partial vulcanization during mixing or extrusion. The network forms during processing.
• Variant D – progressive crosslinking after application. Similar solutions have also been used to reduce cold flow.

Although these materials may be sold under similar names, they are not technically equivalent. When comparing products, it is therefore worth checking the actual crosslinking method and resulting properties rather than relying solely on the trade name.

Common mistakes when trying to reduce migration

“Let’s add more filler”

This may temporarily make oil bleed-out less visible but can also impair tack and substrate wet-out. Adding filler does not eliminate incompatibility between ingredients.

“Let’s use a higher-viscosity oil”

This may help when excessively rapid diffusion is the main problem. It will not deliver the desired effect, however, if the new oil is poorly compatible with the polymer matrix.

“Let’s increase the degree of crosslinking”

More extensive crosslinking may improve creep resistance but simultaneously impair tack, processability and stress relaxation. In a poorly balanced system, it may also promote liquid-phase separation.

“Let’s reduce oil content by 20%”

Reducing oil content may limit migration, but if oil provides substrate wet-out and low-temperature flexibility, the seal may lose essential service properties.

How should the development of a stable butyl compound be planned?

R&D work on butyl tapes or cords should preferably follow this sequence:

1. Define the service-temperature range and the materials the product will contact.
2. Identify every low-molecular-weight ingredient in the formulation.
3. Assess compatibility within the IIR–PIB–plasticizer–tackifying-resin system.
4. Optimize the molecular-weight distribution of the PIB or polybutene phase.
5. Select the type, loading and characteristics of the fillers.
6. Test liquid-phase migration and compound creep separately.
7. If stability at elevated temperature is inadequate, consider controlled partial crosslinking.
8. Validate the product after aging in contact with actual substrates.

This approach requires more work than simply adjusting oil content by a few phr, but improves the prospects of achieving a solution that remains stable over time.

Is a crosslinked compound always better than an uncrosslinked one?

No. The choice should primarily depend on the function and service conditions of the seal.

An uncrosslinked compound is generally a good choice when high tack, easy substrate wet-out, high deformability, re-extrudability and static sealing under low loads are the priorities.

A partially crosslinked compound may perform better where priorities include creep resistance, shape retention, elevated-temperature service, reduced squeeze-out, greater cohesion or sustained mechanical loading.

Commercial examples show that partially crosslinked butyl tapes are primarily marketed for shape stability, creep resistance and performance at higher temperatures, rather than solely for maximum peel strength.

Key principle: compatibility first, crosslinking second

Crosslinking is sometimes regarded as a universal means of limiting migration, but it should not replace proper raw-material selection. A stable butyl compound requires:

• a compatible plasticizer with an appropriately selected molecular weight;
• the correct balance of the entire liquid phase;
• a suitable structure and proportion of IIR and PIB;
• a tackifying resin that does not cause undesirable phase separation;
• a well-selected and uniformly dispersed filler.

Only after these conditions are met does partial crosslinking become an effective tool for controlling rheology and cohesion. It should not be used to conceal formulation errors.

Conclusion

Plasticizer migration in butyl rubber compounds is not simply a matter of excessive oil content. It depends on ingredient compatibility, molecular weight, polymer-matrix mobility, temperature, filler type and dispersion, mixing conditions, time, pressure, contact substrate and degree of crosslinking.

For tapes and cords, it is particularly important to distinguish genuine liquid-phase migration from creep of the entire compound. Uncrosslinked products usually provide excellent tack, wet-out and deformability but are more sensitive to creep and sustained loads. Partial crosslinking can improve dimensional stability and resistance to deformation, but it does not automatically eliminate problems caused by an incompatible plasticizer.

The best results come not from restricting material mobility as much as possible, but from striking the right balance between adhesion, cohesion, plasticity, compatibility and controlled liquid-phase mobility. That balance determines whether the seal retains its properties not only immediately after manufacture, but also during storage and many years of service.