SIS in Hot-Melt Adhesives and Sealants
Styrene-isoprene-styrene (SIS) block copolymer is the base elastomer behind a large share of the world’s hot-melt and pressure-sensitive adhesives. Its combination of high tack, good adhesion, elastic recovery and easy melt processing is hard to match, and it has made SIS the reference polymer for labels, tapes, disposable hygiene products and many sealants and coatings.
This article explains how an SIS-based adhesive is built, what each component contributes, how the adhesive is applied and tested, and the practical rules that keep an SIS bond performing. For background on the polymer itself, see our main article on SIS block copolymers.
Why SIS for Hot-Melt Adhesives
A hot-melt adhesive is applied as a molten blend and sets on cooling. It needs an elastomer that flows when hot, gives a strong, elastic bond when cold, and is compatible with the tackifying resins and oils that make it sticky. SIS meets all three requirements:
- Tack and quick grab. The polyisoprene mid-block is highly compatible with common tackifiers, giving immediate surface tack at low coat weight.
- Elasticity and flexibility. The physical cross-links in the polystyrene domains let the bond stretch and recover, useful on flexible substrates and in hygiene elastics.
- Melt processability. A melt flow rate of 8–14 g/10 min (SIS 1126) gives a viscosity that can be coated, sprayed or slot-die applied on standard equipment.
Anatomy of an SIS Adhesive
A typical SIS hot-melt or pressure-sensitive adhesive contains four or five components. The elastomer provides the elastic network; the other ingredients tune tack, viscosity, ageing and cost.
| Component | Function | Typical chemistry |
|---|---|---|
| SIS elastomer | Elastic network, cohesion | SIS triblock, e.g. SIS 1126 (16% styrene) |
| Tackifier | Tack and adhesion | Rosin esters, C5/C9 hydrocarbon resins, terpene resins |
| Plasticising oil | Soften, reduce melt viscosity | Naphthenic or paraffinic process oils |
| Antioxidant | Protect the unsaturated mid-block | Hindered phenols, phosphites |
| Wax / additives | Open time, set speed, colour, fillers | Paraffin or microcrystalline wax |
Formulators adjust the ratio of elastomer, tackifier and oil to move the adhesive between the three classic regimes: high tack and softness (more tackifier, more oil), balanced performance, and high cohesion and holding power (more elastomer, less oil).
Property Balance: Tack, Peel and Shear
Adhesive performance is usually judged on three axes, and they trade off against one another:
| Property | What it measures | Main drivers |
|---|---|---|
| Tack | Immediate stickiness (probe / loop tack) | Tackifier level, di-block content, oil |
| Peel adhesion | Force to peel from a substrate (180° peel) | Tackifier chemistry, elastomer, coat weight |
| Shear / holding power | Resistance to creep and slip | Bound styrene, molecular weight, elastomer level |
SIS, especially high-di-block grades such as SIS 1126, sits naturally at the tacky, high-peel end of this space. Where greater shear is needed, the formulator raises the elastomer level or the polystyrene content, or blends in a higher-cohesion polymer.
Application Methods
- Slot-die and roll coating for laminates, labels and tapes at controlled coat weight.
- Spray (spiral, swirl, melt-blown) for hygiene construction adhesives, where a light, open pattern gives breathability and softness.
- Wheel and nozzle application for packaging and product assembly.
- Solution coating where a solvent-based adhesive is required.
Because the polyisoprene mid-block is thermally sensitive, the melt should be held at the lowest practical temperature and for the shortest practical time. Overheating degrades tack and colour and can raise the level of volatiles and odour.
Sealants, Mastics and Coatings
Beyond adhesives, SIS is used as an elastic binder in sealants and mastics, where it provides flexible adhesion, low-temperature flexibility and elastic recovery, and as a binder or modifier in coatings where flexibility and adhesion to difficult substrates are required. In these systems the same design logic applies: the elastomer supplies elasticity, and resins and oils adjust softness, adhesion and application properties.
Practical Rules for SIS Adhesives
- Always use an antioxidant. The unsaturated mid-block will otherwise discolour and lose tack under heat and oxygen.
- Keep processing temperatures moderate. Minimise heat history to protect tack and colour.
- Match the tackifier to the mid-block. Rosin esters and hydrocarbon resins are common; compatibility governs tack and clarity.
- Choose the di-block level deliberately. Higher di-block means more tack and lower viscosity; lower di-block means more cohesion.
- Test on the real substrate. Tack, peel and shear must be measured on the actual adherend and at the service temperature.
- Store the polymer sealed and cool. Protect from heat, light and moisture until use.
Frequently Asked Questions
What is SIS used for in adhesives?
SIS is the base elastomer in hot-melt and pressure-sensitive adhesives. It supplies tack, adhesion and elasticity, and is blended with a tackifier, an oil and an antioxidant.
What is the difference between hot-melt and pressure-sensitive adhesive?
A hot-melt adhesive is applied molten and bonds on cooling; a pressure-sensitive adhesive stays permanently tacky and bonds on light contact pressure. SIS is used in both.
Why does an SIS adhesive discolour?
The polyisoprene mid-block is unsaturated and prone to oxidation. Inadequate antioxidant or excessive processing temperature causes yellowing and loss of tack.
How much tackifier should I add?
It depends on the target tack and viscosity; the tackifier level is typically balanced against the elastomer and oil in a short design-of-experiments study. There is no single universal figure.
Can SIS adhesives be used outdoors?
Not without protection. SIS ages under heat, oxygen and UV; for permanent outdoor exposure a hydrogenated polymer such as SEBS is usually preferred.