Choosing the wrong construction sealant is one of the most expensive mistakes on a job site, because the failure shows up months later as a leaking joint, a cracked bond line, or mold growing along a window perimeter. The four sealant chemistries that dominate construction — silicone, polyurethane, MS polymer and acrylic — behave very differently on movement, paintability, UV exposure and substrate adhesion. This guide compares all four on the properties that actually decide joint life, gives a selection matrix you can apply directly, and answers the questions buyers ask most: which sealant resists mold best, which can be painted, and where polyurethane beats silicone. For a focused head-to-head, see our silicone vs polyurethane sealant comparison; this pillar puts all four families side by side.
What a Construction Sealant Has to Do
A sealant is not glue. Its job is to fill a joint and stay flexible while the two sides of that joint expand, contract and shear with temperature and structural movement. The single most important specification is movement capability — the percentage a cured sealant can stretch and compress without losing adhesion or tearing. International sealant performance is classified under ISO 11600 (which sorts sealants into 7.5%, 12.5%, 20% and 25% movement classes) and the North American standard ASTM C920. Before you compare brands, decide the movement class the joint needs; everything else is secondary.
The four properties that separate the chemistries in practice are:
- Movement capability — how much joint movement the cured bead survives.
- Paintability — whether finish coatings will adhere to the cured surface.
- UV and weather durability — how the sealant ages outdoors over 10–20 years.
- Adhesion without primer — which substrates bond directly and which need a primer.
The Four Construction Sealant Families
| Property | Silicone | Polyurethane (PU) | MS Polymer | Acrylic |
|---|---|---|---|---|
| Movement capability | 25–50% (best) | 25% | 25% | 7.5–12.5% (lowest) |
| Paintable | No | Yes | Yes | Yes |
| UV / weather durability | Excellent (20+ yr) | Good (needs UV grade) | Very good | Fair |
| Abrasion / tear strength | Low | High (best) | Medium-high | Low |
| Adhesion without primer | Glass, metal, ceramic | Concrete, wood (often needs primer) | Broadest range | Porous only |
| Mold / mildew resistance | Best (with fungicide grade) | Moderate | Good | Poor |
| Relative cost | Medium | Medium-high | High | Low |
Silicone Sealant
Silicone is the highest-movement, most weather-durable chemistry and the default for glazing, sanitary joints and curtain walls. It bonds to glass, glazed ceramic and metal without a primer and holds its elasticity for 20+ years outdoors. Its two limitations are decisive: it cannot be painted (no coating adheres to the cured silicone surface), and standard grades have low tear strength so it is unsuitable for traffic-bearing or abrasion-exposed joints. For wet areas, specify a fungicide-treated sanitary grade — this is the formulation that resists the black mold that ruins bathroom and kitchen perimeters. Desay supplies a neutral-cure S168 silicone sealant for glass and construction joints.
Polyurethane (PU) Sealant
Polyurethane is the toughest sealant: high tear and abrasion resistance, excellent adhesion to concrete and masonry, and it is paintable. That combination makes it the standard for floor joints, expansion joints, precast concrete and any joint that takes foot or vehicle traffic. Its trade-offs are UV sensitivity (outdoor joints need a UV-stabilised grade or a paint topcoat) and a shorter outdoor service life than silicone. Where a joint moves and gets walked or driven on, polyurethane beats silicone every time.
MS Polymer (Modified Silane) Sealant
MS polymer is the hybrid that tries to combine silicone's weatherability with polyurethane's paintability and toughness. It is isocyanate-free and solvent-free, bonds to the widest range of substrates without primer, is paintable, and weathers well. The catch is cost — MS polymer is typically the most expensive of the four. It is the right choice for high-spec facade and marine work where a single product must paint, weather and bond to mixed substrates, but it is over-specified for routine interior joints.
Acrylic Sealant
Water-based acrylic (decorator's caulk) is the low-cost, paintable filler for low-movement interior gaps — skirting, trim and crack filling. It paints easily and cleans up with water, but its low movement capability (7.5–12.5%) and poor wet-area performance rule it out of any joint that moves or gets wet. Specify acrylic only for static interior joints that will be painted.
Selection Framework
Work through these four questions in order:
- Does the joint move? High movement (glazing, curtain wall) → silicone. Moderate movement on a traffic surface → polyurethane. Low movement interior → acrylic.
- Must it be painted? If yes, eliminate silicone — choose polyurethane, MS polymer or acrylic.
- Is it outdoors or wet? Outdoor weathering → silicone or MS polymer. Wet/sanitary → fungicide silicone.
- Does it take traffic or abrasion? If yes → polyurethane for tear strength.
Joint design matters as much as chemistry: maintain the correct width-to-depth ratio (typically 2:1 for elastic sealants), use a backer rod to control depth and prevent three-sided adhesion, and respect the manufacturer's joint-width limits for the movement class. A correctly specified sealant in a badly designed joint still fails. Our waterproof adhesive and sealant guide covers joint detailing for wet construction in depth.
Common Procurement Mistakes
- Buying on price without a movement class — a cheap acrylic in a 25% movement joint will split within a season. Specify the ISO 11600 / ASTM C920 class first.
- Using silicone where the joint must be painted — the most common callback. No paint adheres to cured silicone; switch to PU or MS polymer.
- Skipping the fungicide grade in wet areas — standard silicone grows mold in bathrooms and kitchens. Specify a sanitary/fungicidal grade for any wet perimeter.
- Ignoring primer requirements — polyurethane on concrete or porous stone often needs a primer for durable adhesion; test the substrate before bulk ordering.
- Not checking shelf life and cure system — neutral-cure vs acetoxy-cure silicone behave differently on metal and stone. Confirm the cure chemistry and request a TDS and COA per batch.
Related Technical Guides
- Silicone vs Polyurethane Sealant — the focused head-to-head on the two most-specified chemistries.
- Waterproof Adhesive & Sealant for Construction — joint detailing and waterproofing for wet builds.
- Construction Adhesive Types — when you need a structural adhesive rather than a flexible sealant.
Desay Industrial supplies silicone sealant, manual sealing glue and transparent waterproof sealant for construction and industrial joints, in industrial packaging (MOQ 500 kg) with 15-day delivery to 60+ countries. All products carry ISO 9001, SGS and REACH certification, with TDS and COA supplied per batch. Contact our technical team for samples and a movement-class recommendation matched to your joint design.