SIS vs SBS: An Overview
SIS (styrene-isoprene-styrene) and SBS (styrene-butadiene-styrene) are two of the most widely used styrenic block copolymers. Both are thermoplastic elastomers with the same A–B–A architecture — two hard polystyrene end blocks flanking a soft rubber mid-block — but they differ in that mid-block: SIS uses polyisoprene, SBS uses polybutadiene. That single difference drives nearly all of their behavioural differences, and it is the key to choosing between them.
This guide compares the two polymers on tack, cohesion, ageing, processing and cost, and gives practical guidance on which to select for adhesives, sealants, coatings and polymer modification. For the polymer chemistry in more depth, see our main article on SIS block copolymers.
At a Glance
| Property | SIS | SBS |
|---|---|---|
| Mid-block | Polyisoprene (unsaturated) | Polybutadiene (unsaturated) |
| End blocks | Polystyrene | Polystyrene |
| Tack | Excellent | Good |
| Adhesion | Excellent | Good |
| Cohesion / strength | Good | Often higher |
| Heat, oxygen and UV resistance | Moderate | Moderate (slightly better) |
| Low-temperature flexibility | Excellent (mid-block Tg approx. -55 °C) | Good (mid-block Tg approx. -85 °C, but harder to tackify) |
| Compatibility with tackifiers | Very good | Good |
| Typical cost | Intermediate | Lower |
| Main use | Hot-melt and PSA, tapes, labels, hygiene | Adhesives, bitumen and plastics modification |
Why the Mid-Block Matters
The physical cross-linking that gives both polymers their elasticity comes from the polystyrene domains. The mid-block, however, controls the softness, the tack and the compatibility with the tackifier resins and oils that surround it.
Polyisoprene (SIS) is highly compatible with the rosin esters and hydrocarbon resins used as tackifiers, which lets an SIS adhesive develop strong, immediate tack and quick grab at low coat weight. This is the reason SIS dominates pressure-sensitive adhesives for labels, tapes and disposable hygiene products, where a light, aggressive, fast-tacking adhesive is essential.
Polybutadiene (SBS) is somewhat less compatible with common tackifiers and tends to give a firmer, more cohesive bond. SBS is consequently a strong general-purpose choice for adhesive and sealant systems, and it is the workhorse for modifying bitumen and plastics. Where a higher-modulus, tougher polymer is acceptable, SBS usually costs less than SIS.
Tack, Cohesion and Adhesion
The practical difference most formulators notice is the tack/cohesion balance. SIS-based adhesives tend to be softer, tackier and more conformable, with excellent adhesion to low-energy and difficult substrates. SBS-based adhesives tend to be firmer and more cohesive, with better shear and holding power but less immediate tack.
The di-block content of the grade amplifies this. A higher di-block fraction (such as the 50% di-block of SIS 1126) reduces melt viscosity and increases tack and adhesion, pushing the balance further toward aggressive pressure-sensitive performance. A lower di-block fraction raises cohesion.
Ageing, Heat and UV Resistance
Both SIS and SBS have unsaturated mid-blocks and therefore both are sensitive to oxidation, heat and ultraviolet light. Both always require an antioxidant package, and both are outperformed in weathering by the hydrogenated relative SEBS (styrene-ethylene-butylene-styrene). Between the two, SBS’s polybutadiene mid-block is sometimes described as slightly more resistant than polyisoprene, but the difference is modest and both are unsuitable for permanent outdoor exposure without protection or hydrogenation.
The practical rule: for long-term outdoor, high-temperature or UV-exposed service, choose SEBS; for indoor or protected applications where tack and adhesion dominate, SIS remains the better-balanced choice, and SBS a cost-effective alternative.
Processing and Cost
Both polymers are processed from the melt or from solution on conventional equipment. Their melt viscosities are set largely by molecular weight (reflected in melt flow rate and di-block content) and by the formulation. SBS is generally the lower-cost material, which is why it is favoured for large-volume applications such as bitumen modification, where a good balance of elasticity and price matters more than maximum tack.
| Sector | Common choice | Reason |
|---|---|---|
| Pressure-sensitive labels and tapes | SIS | High tack, adhesion, fast grab |
| Disposable hygiene adhesives | SIS | Elasticity, softness, compatibility |
| General hot-melt adhesives | SIS or SBS | Balance of tack and cohesion vs cost |
| Sealants and mastics | SIS or SBS | Elastic recovery, adhesion |
| Bitumen / asphalt modification | SBS (often), SIS (selected) | Elasticity, low-temperature flexibility, cost |
| Plastics modification | SBS or SEBS | Toughness, impact resistance |
| Outdoor / high-temperature TPE | SEBS | Oxidation and UV resistance |
Which Should You Choose
- Choose SIS when maximum tack, adhesion and softness are wanted — labels, tapes, hygiene and other pressure-sensitive systems — and when cost is secondary to performance.
- Choose SBS when a firmer, more cohesive bond is acceptable and lower cost or higher cohesion is prioritised, especially for adhesives, sealants and polymer or bitumen modification.
- Choose SEBS when resistance to heat, oxygen and ultraviolet light is essential and tack is not the primary requirement.
Frequently Asked Questions
Is SIS or SBS better for adhesives?
For tack and pressure-sensitive performance, SIS is generally better. SBS is a lower-cost option where more cohesion is acceptable. The right choice depends on the tack/cohesion/ageing balance you need.
Can I use SBS instead of SIS?
Often yes for general adhesives and sealants, but expect less tack and a firmer feel. For labels, tapes and hygiene adhesives, SIS is usually preferred.
Are SIS and SBS both thermoplastic elastomers?
Yes. Both are A–B–A styrenic block copolymers that behave as elastomers at room temperature and flow as melts when heated, because cross-linking is physical rather than chemical.
Which resists weathering better?
Neither SIS nor SBS resists weathering well because their mid-blocks are unsaturated. For outdoor and high-temperature use, SEBS is the better choice.
What is the CAS number of SIS?
The styrene-isoprene-styrene block copolymer is assigned CAS 25038-32-8.