Buying SIS Block Copolymer: What Matters
SIS block copolymer (CAS 25038-32-8) is sold against a technical specification, and two grades with the same trade name can behave very differently in the same adhesive or compound. Buying well means matching the grade to the application and confirming the numbers that govern tack, cohesion, melt viscosity and colour.
This guide explains the parameters that define an SIS grade, how to read a certificate of analysis (COA), the questions to ask a supplier, and the mistakes that cost formulators time and money. For background on the polymer, see our main article on SIS block copolymers.
Step 1: Define the Application
Start from the end use, because it determines the grade family.
| Application | Grade preference | Why |
|---|---|---|
| Pressure-sensitive adhesives | Low styrene, higher di-block | Maximum tack and adhesion |
| General hot-melt adhesives | Low to medium styrene | Balance of tack and cohesion |
| Sealants and coatings | Low to medium styrene | Elasticity and low-temperature flexibility |
| Bitumen / plastics modification | Medium to high styrene | Strength and cohesion |
Step 2: Read the Specification, Not Just the Name
A styrenic block copolymer specification contains the numbers that determine performance. The table below lists the parameters to check, using the SIS 1126 grade as an example.
| Parameter | Why it matters | SIS 1126 |
|---|---|---|
| Bound styrene | Controls softness, tack and cohesion | 16.0 ± 2.0 wt% |
| Di-block content | Controls tack and melt viscosity | 50.0 ± 3.0 wt% |
| Melt flow rate | Controls processing and flow | 8.0–14.0 g/10 min |
| Volatility | Odour and end-use compliance | ≤ 0.70 wt% |
| Ash content | Residual inorganics / cleanliness | ≤ 0.20 wt% |
| Yellow index | Colour; important for clear, light adhesives | ≤ 6.0 |
| Tensile strength | Cohesive strength | ≥ 4.0 MPa |
| Elongation at break | Softness and extensibility | ≥ 1100% |
For adhesive buyers the bound styrene, di-block content and melt flow rate are the three decisive figures. Colour (yellow index) and volatility matter where the finished product must stay clear and low-odour.
Step 3: Match the Physical Form
SIS is supplied as pellets, crumb or powder. Pellet or crumb form is the norm for melt compounding; the choice affects feeding, dust and handling. Confirm the form and the packaging (typically 25 kg bags or larger units) that suits your plant.
Step 4: Understand Consistency and Documentation
Because the polymer is a compound feedstock, batch-to-batch consistency in bound styrene, di-block and melt flow rate is critical. Ask for a per-batch COA and, if melt flow rate or colour is critical to your process, a recent batch history. For regulated end uses, check whether the grade carries the necessary food-contact or other statements; do not assume a general-purpose grade is compliant.
Supplier Checklist
- Grade definition. Bound styrene, di-block content, melt flow rate, form.
- COA per batch. Will a certificate of analysis accompany each shipment, and can you see a recent example?
- Documentation. SDS, REACH and, where required, food-contact or other statements.
- Consistency. How are the specification parameters controlled batch to batch?
- Physical form and packaging. Pellets, crumb or powder; unit sizes offered.
- Antioxidant. What antioxidant is included and is it adequate for your process?
- Lead time, MOQ and Incoterm. Confirm commercial terms.
Common Buying Mistakes
- Buying on price alone. A cheaper grade with lower di-block or higher volatility may underperform or need reformulation.
- Ignoring di-block. Di-block strongly affects tack and melt viscosity; overlooking it disrupts the adhesive design.
- Assuming grades are interchangeable. Adhesive, sealant and modification grades differ materially.
- Not bench-testing. Viscosity, tack and colour should be confirmed in your own compound before scaling up.
- Neglecting storage. SIS should be kept sealed, cool and dark; the unsaturated mid-block ages under heat and light.
How PolyBlueChem Can Help
PolyBlueChem supplies SIS block copolymer (CAS 25038-32-8, grade SIS 1126), a clear, linear styrene-isoprene-styrene copolymer with 16% bound styrene, for adhesives, sealants, coatings and compound formulations. We can provide current specifications and a COA, discuss the grade that fits your application, and quote packaging and shipping options. To request specifications, a sample or a quotation, contact the PolyBlueChem team.
Frequently Asked Questions
What should I look for when buying SIS?
Match the grade to the application, then check bound styrene, di-block content and melt flow rate as the decisive parameters, plus colour, volatility and documentation. Confirm the supplier provides a per-batch COA.
What is the CAS number of SIS block copolymer?
25038-32-8 (polystyrene-block-polyisoprene-block-polystyrene).
Does di-block content really matter?
Yes. It strongly influences tack and melt viscosity, so it is one of the first parameters to specify for adhesive grades.
What documents should come with a shipment?
At minimum a certificate of analysis and a safety data sheet; for regulated uses, the relevant compliance statements as well.
How should SIS be stored after purchase?
Keep it sealed, cool, dry and out of direct sunlight, away from heat and ignition sources, to protect the unsaturated mid-block from ageing.