Introduction to SIS Block Copolymers
SIS (styrene-isoprene-styrene) is a thermoplastic elastomer built from three blocks: two hard polystyrene end blocks and a soft polyisoprene middle block. The material behaves like a cross-linked rubber at room temperature, yet it melts and flows when heated, so it can be processed on ordinary plastics equipment and needs no vulcanisation. This combination — rubber-like elasticity without chemical cross-linking — has made SIS one of the most important polymers in the adhesives industry.
The polymer is assigned CAS 25038-32-8 (polystyrene-block-polyisoprene-block-polystyrene). Commercial grades are described by their bound (polystyrene) content, their molecular weight, their di-block content and their melt flow rate. A typical adhesive grade contains about 15–25% polystyrene; the grade discussed throughout this article, SIS 1126, contains 16% bound styrene and is a clear, linear block copolymer supplied by PolyBlueChem.
This article reviews the chemistry and morphology of SIS, its physical and processing properties, how it is produced, the grades and specifications traded, how it compares with SBS and SEBS, and its principal applications in hot-melt adhesives, sealants, coatings and bitumen modification.
Chemical Structure and Morphology
SIS is a linear triblock copolymer with the architecture polystyrene–polyisoprene–polystyrene, often written as S–I–S or A–B–A. The two polystyrene blocks are incompatible with the polyisoprene mid-block, so on cooling the material self-assembles into a two-phase structure: glassy polystyrene domains dispersed in a continuous, rubbery polyisoprene matrix. The polystyrene domains act as physical cross-links — they anchor the rubbery chains and give the material its elastic recovery — but they soften when heated above the polystyrene glass transition temperature of roughly 100 °C, which is why the polymer can be melt-processed and recycled.
The polyisoprene mid-block is the source of the material’s softness and tack. It is unsaturated (it retains carbon-carbon double bonds from the isoprene monomer), which makes SIS particularly compatible with the rosin and hydrocarbon tackifiers used in adhesives and gives it excellent tack and adhesion. The same unsaturation, however, means SIS is more sensitive to oxidation and ultraviolet light than its hydrogenated relative SEBS, and antioxidants are always added to protect it.
Key structural features
- Architecture: linear A–B–A triblock (polystyrene–polyisoprene–polystyrene).
- End blocks: polystyrene, forming rigid domains that act as physical cross-links (Tg approx. 100 °C).
- Mid-block: polyisoprene, a soft, unsaturated rubber (Tg approx. -55 °C).
- Bound styrene: typically 14–25% for adhesive grades; 16% for SIS 1126.
- Di-block content: the fraction of two-block (S–I) molecules rather than three-block (S–I–S); it strongly influences tack, viscosity and adhesion.
- Molecular formula: nominal (C8H8)x–(C5H8)y–(C8H8)x, from styrene and isoprene monomers.
Physical and Thermal Properties
SIS is a low-density, translucent to clear elastomer. Its most important physical properties are the two-phase morphology described above, a low rubber glass transition temperature, and a low melt viscosity when heated. The table below gives indicative values and the specification of SIS 1126.
| Property | Typical / SIS 1126 value |
|---|---|
| Appearance | Clear to translucent pellets or crumb; white as supplied |
| Density | Approx. 0.92–0.94 g/cm³ |
| Bound styrene | 16.0 ± 2.0 wt% (SIS 1126) |
| Di-block content | 50.0 ± 3.0 wt% (SIS 1126) |
| Glass transition (polyisoprene mid-block) | Approx. -55 °C |
| Softening (polystyrene domain) | Approx. 90–100 °C |
| Volatility | ≤ 0.70 wt% |
| Ash content | ≤ 0.20 wt% |
| Yellow index | ≤ 6.0 |
| Melt flow rate | 8.0–14.0 g/10 min |
| Tensile strength | ≥ 4.0 MPa |
| Elongation at break | ≥ 1100% |
The very high elongation at break (at least 1100%) and modest tensile strength reflect a soft, highly extensible elastomer with a low polystyrene content. The yellow index and low volatility and ash figures indicate a clean, colour-stable polymer suitable for adhesives where colour and odour matter.
Melt Rheology and Processing
Above the softening point of the polystyrene domains, SIS flows as a viscous melt, and it is usually processed from the melt. The melt flow rate (MFR, 8.0–14.0 g/10 min for SIS 1126) is the key processing indicator: a higher MFR means lower melt viscosity and easier flow, while a lower MFR means higher molecular weight and greater cohesive strength. Adhesive formulators balance MFR against the strength and creep resistance the finished bond requires.
SIS is processed by:
- Hot-melt compounding and coating: blended with tackifiers, plasticising oils and antioxidants, then coated or sprayed onto substrates.
- Extrusion and moulding: extruded into film, tape or profiles, or injection-moulded where elastic, soft parts are needed.
- Solution blending: dissolved in hydrocarbon solvents for solvent-based adhesives and coatings.
Because the polyisoprene mid-block is thermally sensitive, processing temperatures and residence times should be kept moderate, and adequate antioxidant is essential to avoid discolouration, cross-linking or loss of tack.
Production Process
SIS is produced by living anionic polymerisation in a hydrocarbon solvent, most commonly cyclohexane, using an organolithium initiator such as sec-butyllithium. The process builds the blocks one after another with excellent control over molecular weight and block length, which is why styrenic block copolymers can be made to precise specifications.
| Stage | Purpose | Notes |
|---|---|---|
| 1. Purification of raw materials | Remove impurities that would terminate the living chain | Styrene, isoprene and solvent are dried and purified |
| 2. Initiation and first styrene block | Form the first polystyrene block | Organolithium initiator; controlled molecular weight |
| 3. Isoprene mid-block | Grow the rubbery polyisoprene centre block | Sequential addition of isoprene monomer |
| 4. Second styrene block / coupling | Complete the triblock | Sequential styrene addition, or coupling of two S–I diblock chains; coupling route can leave a controlled di-block fraction |
| 5. Termination and antioxidant | Stop the polymerisation and stabilise the polymer | Antioxidant added before isolation |
| 6. Recovery and finishing | Isolate the solid polymer | Solvent stripping and drying; pelletising or crumbing |
The di-block content is a deliberate outcome of the production route. A sequential process gives a nearly pure triblock, whereas a coupling process can leave a controlled fraction of un-coupled S–I diblock. Di-block acts as a “diluent” that lowers melt viscosity and increases tack and adhesion, which is why grades intended for pressure-sensitive adhesives are often specified with a definite di-block content — 50% in the case of SIS 1126.
Grades and Specifications
SIS grades are defined mainly by their polystyrene content, molecular weight (or melt flow rate), di-block content and physical form. The table below shows how these variables map to application.
| Grade type | Bound styrene | Typical use |
|---|---|---|
| Low styrene (15–20%) | Low | Pressure-sensitive and hot-melt adhesives; soft, tacky, high elongation (SIS 1126 is here) |
| Medium styrene (20–30%) | Medium | Adhesives, sealants and coatings needing more cohesion |
| High styrene (30–45%) | High | Compounds and plastics modification; greater strength and hardness |
| High di-block | Variable | High-tack PSAs; lower melt viscosity |
| Low di-block | Variable | Higher cohesion; less tack |
The full specification of SIS 1126 is: appearance white; bound styrene 16.0 ± 2.0 wt%; di-block 50.0 ± 3.0 wt%; volatility ≤ 0.70 wt%; ash ≤ 0.20 wt%; yellow index ≤ 6.0; melt flow rate 8.0–14.0 g/10 min; tensile strength ≥ 4.0 MPa; elongation at break ≥ 1100%.
How SIS Compares with SBS and SEBS
SIS belongs to the family of styrenic block copolymers, alongside SBS (styrene-butadiene-styrene) and SEBS (styrene-ethylene-butylene-styrene, the hydrogenated form of SBS). Choosing among them is one of the most common decisions in adhesive and compound formulation.
| Property | SIS | SBS | SEBS |
|---|---|---|---|
| Mid-block | Polyisoprene (unsaturated) | Polybutadiene (unsaturated) | Ethylene/butylene (saturated) |
| Tack and adhesion | Excellent | Good | Moderate |
| Heat and UV / oxidation resistance | Moderate | Moderate | Excellent |
| Softness and elasticity | High | Good | Good |
| Main use | Hot-melt and PSA, sealants, tapes | Adhesives, bitumen and plastics modification | Weather-resistant TPE, automotive, medical |
| Relative cost | Intermediate | Lower | Higher |
In short, SIS is chosen when tack and adhesion matter most, as in labels, tapes and hygiene adhesives. SBS is a versatile workhorse for adhesives and polymer modification where cost is important. SEBS is selected where resistance to heat, oxygen and ultraviolet light is essential. A more detailed comparison is given in the related reading below.
Applications Overview
SIS is used as an ingredient in formulating adhesives, sealants and coatings, and as a modifier of bitumen and thermoplastics. The table summarises the main application areas.
| Application | Role of SIS | Typical properties exploited |
|---|---|---|
| Hot-melt pressure-sensitive adhesives | Base elastomer | Tack, adhesion, elasticity, low melt viscosity |
| Labels, tapes and films | Base elastomer | Tack, cohesive strength, ageing (with antioxidant) |
| Hygiene and disposable products | Construction and elastic adhesives | Elasticity, softness, compatibility with additives |
| Sealants and mastics | Elastomeric binder | Elastic recovery, adhesion, flexibility at low temperature |
| Coatings | Binder / modifier | Flexibility, adhesion, film formation |
| Bitumen (asphalt) modification | Modifier | Elasticity, low-temperature flexibility, cohesion |
| Thermoplastic modification | Impact / flexibilising modifier | Toughness, elasticity, compatibility |
| Compounds and masterbatches | Base elastomer | Formulation flexibility, oil and tackifier acceptance |
Hot-Melt Pressure-Sensitive Adhesives
The single largest use of SIS is in hot-melt pressure-sensitive adhesives (HMPSAs). A typical HMPSA contains an SIS elastomer, one or more tackifying resins, a plasticising oil, an antioxidant and, optionally, a wax or other additive. The elastomer provides elasticity and cohesive strength; the tackifier makes the surface sticky; the oil softens the blend and lowers viscosity; the antioxidant protects the unsaturated mid-block from ageing.
| Component | Function | Typical examples |
|---|---|---|
| SIS elastomer | Elastic network, cohesion, elasticity | SIS 1126 and similar grades |
| Tackifier | Tack and adhesion | Rosin esters, C5 and C9 hydrocarbon resins, terpene resins |
| Plasticising oil | Soften, reduce viscosity | Naphthenic or paraffinic process oils |
| Antioxidant | Protect against heat, oxygen and UV | Hindered phenols, phosphites |
| Wax / additives | Adjust open time, set speed, colour | Paraffin or microcrystalline wax |
Because the polyisoprene mid-block is highly compatible with tackifier resins, SIS-based HMPSAs achieve excellent tack and quick grab at low coat weight, which is why they dominate in labels, tapes and disposable hygiene products. The trade-off is that SIS-based adhesives need effective antioxidant protection to resist discolouration and loss of tack in storage and use.
Sealants, Coatings and Compounding
In sealants and mastics, SIS provides an elastic, adhesive binder that remains flexible at low temperature and recovers after deformation. In coatings, SIS is used as a binder or modifying polymer where flexibility and adhesion to difficult substrates are needed. The polymer’s compatibility with oils and resins makes it easy to compound: formulators can adjust softness, tack and viscosity by changing the oil and tackifier balance without changing the elastomer, which is a key reason SIS is supplied as a compounding feedstock rather than a finished product.
Bitumen and Thermoplastic Modification
SIS can be used as a modifier of bitumen (asphalt), improving elasticity, low-temperature flexibility and cohesion in asphalt binders and in some roofing and waterproofing formulations. For heavy-duty or highly weather-exposed bitumen applications, the more oxidation-resistant SBS or SEBS may be preferred, but SIS offers attractive adhesion and elasticity in appropriate systems. As a thermoplastic modifier, SIS is blended into polystyrene, polyolefins and other thermoplastics to increase toughness, flexibility and impact resistance.
Formulation and Handling Guide
- Antioxidant is essential. The unsaturated mid-block is prone to oxidation. Always use an adequate antioxidant package and store the polymer sealed, cool and dark.
- Control processing temperature. Keep melt temperatures and residence times moderate; excessive heat degrades tack and colour.
- Balance di-block for tack. Higher di-block content raises tack and lowers viscosity; lower di-block content raises cohesion. Choose the grade to match the job.
- Match the tackifier to the mid-block. Rosin esters and C5/C9 hydrocarbon resins are commonly used with SIS; compatibility affects tack, clarity and ageing.
- Compatibilise blends. When SIS is used to modify bitumen or thermoplastics, check compatibility and phase behaviour at the use temperature.
- Storage. Keep in sealed packaging, cool, dry and away from heat, sunlight and ignition sources.
Quality, Regulatory and Safety
SIS block copolymers are bulk industrial polymers supplied against a technical specification rather than a food or pharmaceutical monograph. Quality is defined by the parameters in the specification table — bound styrene, di-block, volatility, ash, yellow index, melt flow rate and tensile properties — and a certificate of analysis (COA) should accompany each shipment. Key regulatory and safety points:
- Residual monomers and volatiles. Control of residual styrene and total volatiles (≤ 0.70 wt% for SIS 1126) is important for odour and end-use compliance; confirm limits with the supplier for sensitive applications.
- Food contact and medical uses. Styrenic block copolymers are used in some food-contact and medical applications, but the specific grade and formulation must comply with the relevant regulation; do not assume a general-purpose grade is compliant.
- Handling. The polymer is supplied as pellets or crumb and is not hazardous as such, but it should be handled with normal good practice; at processing temperature, avoid breathing any fumes and provide adequate ventilation.
- Documentation. Request the SDS, COA and, where needed, statements on REACH, food contact, heavy metals and residual monomers.
Procurement and Sourcing Guide
When purchasing SIS block copolymer, the specification — not the trade name — determines performance. A practical checklist for buyers:
- Bound styrene. Confirm the polystyrene content matches your target softness and tack. SIS 1126 is 16%.
- Di-block content. This drives tack and melt viscosity; specify it if your formulation depends on it (SIS 1126 is 50% di-block).
- Melt flow rate. Match MFR to your processing (8.0–14.0 g/10 min for SIS 1126).
- Colour and volatiles. Low yellow index and volatility matter for clear, low-odour adhesives.
- Physical form. Pellets versus crumb affects feeding and handling.
- Documentation and consistency. Ask for a per-batch COA and recent batch history if MFR or colour is critical.
PolyBlueChem can supply SIS block copolymer (CAS 25038-32-8, grade SIS 1126). To request current specifications, a COA or a quotation, contact the PolyBlueChem team.
Selecting a Grade and Troubleshooting
Because SIS is almost always used in a blend, the grade choice and the blend design go together. The table below links common performance complaints to the grade or formulation variable that usually governs them, which is a useful starting point when a formulation does not meet its target.
| Requirement or problem | Variable to adjust | Direction |
|---|---|---|
| Not tacky enough | Di-block content; tackifier level; oil level | Higher di-block; more tackifier; less oil |
| Too soft / poor cohesion | Bound styrene; molecular weight (lower MFR) | Higher styrene; lower melt flow rate |
| Melt viscosity too high | Melt flow rate; di-block; oil | Higher MFR; more di-block; more oil |
| Discolours on ageing | Antioxidant package; processing temperature | Stronger antioxidant; lower temperature |
| Poor low-temperature flexibility | Plasticiser and tackifier selection | Softer oil; compatible tackifier |
| Adhesion to a difficult substrate | Tackifier chemistry; SIS vs SBS/SEBS | Match tackifier polarity; consider SIS |
| Weak heat resistance | Polymer family | Consider SEBS for high-temperature ageing |
A practical workflow is to fix the elastomer grade from the required softness (bound styrene) and strength (molecular weight / MFR), then tune tack and viscosity with the tackifier and oil. Only the elastomer grade is fixed at purchase; the rest is blending, which is why SIS is supplied as a feedstock and evaluated in the customer’s own compound.
Testing and Quality Control
Both the polymer and the finished adhesive are controlled by standard tests. For the polymer, the specification parameters — bound styrene, di-block, volatile content, ash, yellow index, melt flow rate, tensile strength and elongation — are measured on the incoming material and confirmed against the certificate of analysis. For the adhesive or compound, the usual checks include:
- Melt viscosity at the coating temperature (e.g. Brookfield, 160–180 °C), to confirm the melt can be coated or sprayed.
- Rheology and softening point, to characterise the tackifier/elastomer balance.
- Tack and peel adhesion (probe tack, loop tack, 180° peel) on the target substrate.
- Shear and holding power, reflecting cohesion and the polystyrene content.
- Colour and ageing, checked by heat-age and yellow-index measurements to confirm antioxidant adequacy.
Because the polymer is a feedstock, arriving at a robust specification usually involves a short design-of-experiments exercise across elastomer level, tackifier type and oil level, followed by aged testing to confirm stability.
Frequently Asked Questions
What is SIS and what is its CAS number?
SIS is a styrene-isoprene-styrene triblock copolymer, a thermoplastic elastomer; the block copolymer is assigned CAS 25038-32-8. For the product specification see the SIS product page.
What does 16% bound styrene mean?
It is the polystyrene content by weight. A low styrene content gives a soft, tacky, highly elastic material (elongation above 1100% for SIS 1126), ideal for adhesives; higher styrene gives more strength and hardness.
What is di-block content and why does it matter?
Di-block is the fraction of two-block S–I molecules. A higher di-block content reduces melt viscosity and increases tack and adhesion, which is valuable in pressure-sensitive adhesives. SIS 1126 has a high 50% di-block content.
Is SIS better than SBS for adhesives?
For tack and pressure-sensitive adhesion, SIS usually performs better because polyisoprene is more compatible with tackifiers. SBS can be a lower-cost option where greater cohesion is acceptable. The choice depends on the balance of tack, cohesion and ageing resistance required. See the dedicated comparison in the related reading.
Can SIS be recycled or reprocessed?
Because cross-linking is physical rather than chemical, SIS can be re-melted and reprocessed like a thermoplastic, though repeated heat history should be limited to protect the unsaturated mid-block.
How should SIS be stored?
In sealed packaging, cool, dry and away from heat, sunlight and ignition sources. The unsaturated mid-block benefits from antioxidant protection and protection from prolonged heat and light.
What is SIS used for?
Mainly hot-melt and pressure-sensitive adhesives (labels, tapes, hygiene products), plus sealants, coatings, bitumen and thermoplastic modification, and general compound formulations.
Does SIS need a tackifier?
On its own, SIS is elastic but not strongly tacky at the surface. In adhesives it is blended with a tackifying resin, an oil and an antioxidant to develop the desired tack, viscosity and ageing resistance.
References and Disclaimer
References. Technical data for styrene-isoprene-styrene block copolymers and grade SIS 1126; general literature on living anionic polymerisation of styrenic block copolymers; standard formulation practice for hot-melt pressure-sensitive adhesives. Specifications quoted reflect the SIS 1126 grade and should be confirmed against the current certificate of analysis for each shipment.
Disclaimer. This article is provided for general technical information only. Statements about properties and applications are indicative and do not constitute a specification or a warranty of fitness for any particular use. Users are responsible for confirming regulatory status, safe handling and suitability for their own application.