Silicone to MS Polymer Sealant on a Line: What to Check First?

This guide covers production lines that apply one-component sealant to parts, often parts painted later. It does not cover building expansion joints, two-component grades or glazing, where the joint design and the specification govern the choice. Values from technical data sheets are typical values for named grades, not specification limits for every product.

Which conditions decide whether to move a line from silicone to MS polymer sealant?

Six line conditions decide whether a silicone to MS polymer sealant switch goes ahead now, after a trial, or not at all: substrates, service temperature, painting after sealing, takt, moisture control in storage and dispensing, and customer approval. The table puts each condition next to the likely outcome and the check that settles it.

The deciding row on most lines is painting after sealing: if no part is painted, the switch rests on adhesion and temperature alone, and silicone often stays.

Read the table in order of cost. A temperature limit or a customer approval can stop the switch before any trial, so check those rows first and on paper. Adhesion, takt and moisture control need parts, equipment and time on the line, so they come next, in one planned trial rather than in production. Write down the result you expect for each row before the trial starts, for example cohesive failure in peel, handling within the current takt, and no cured plugs after a weekend stop. A switch that passes every row can go ahead; a single failed row means either a different MS polymer grade, a process change such as a primer, or keeping silicone on that line.

Does MS polymer sealant need a primer where silicone did not?

MS polymer sealant may need a primer on some substrates where silicone did not, so the primer question is answered per substrate by trial, not by sealant family. One MS polymer data sheet states that "depending on the surface it can be necessary to roughen the surface or to use a primer/adhesion promoter to provide best adhesion" and lists no adhesion to polyethylene, polypropylene and PTFE (Henkel, 2021). A general-purpose silicone data sheet reports unprimed adhesion on many substrates such as glass, metals and most common engineering plastics, and still recommends its primer for maximum adhesion (Dow, 2023).

Maker literature disagrees on silicone adhesion. An MS polymer maker describes silicone adhesion to substrates as poor (Kaneka Belgium, 2026), while the silicone maker reports unprimed adhesion on many substrates. Both statements come from sellers, so your own test decides.

ASTM C794-18(2022), the adhesion-in-peel test for elastomeric joint sealants, is written for building construction sealants, but its peel set-up suits a substrate and primer screen on line parts. ASTM notes that many sealant makers use it for sealant and primer combinations on unusual or proprietary substrates, and the method requires results to indicate whether failure is primarily adhesive or cohesive (ASTM C794-18(2022)). Peel each production substrate, including painted, powder-coated and plastic parts, with and without primer. Cohesive failure inside the sealant is the usual target, though ASTM notes it is not necessarily better than adhesive failure if the adhesive value is sufficient for the application. The same ASTM page warns that this test alone is not appropriate for comparing the overall performance of different sealants.

How does MS polymer cure speed change line takt and handling time?

MS polymer cure speed changes line takt mainly through skin time and the humidity in your shop, because through-cure rates in maker data are close to those of silicone. One MS polymer sealant forms a skin in about 10 to 40 min at 23 °C and 50% relative humidity and cures about 4 mm per 24 h (Henkel, 2021). A second MS polymer grade skins in 25 min and cures about 3 mm per 24 h in the same climate, with thicker layers needing more time (MIPA, 2021). A general-purpose silicone lists a typical skin-over time of 7 min and tack-free time of 20 min, and cures about 3 mm deep in 24 h at room temperature and 50% relative humidity (Dow, 2023).

Two consequences follow for the line. First, the tooling window can grow: a bead that skinned in 7 min with silicone may stay open several times longer with an MS polymer grade, which helps tooling but delays dust-free handling. Second, the same data sheet notes that low temperature and low moisture retard curing, and that cure time also depends on joint depth (Henkel, 2021). A winter shift at low humidity can therefore miss the handling time a summer trial showed.

Measure skin and handling time on real parts at the lowest humidity your shop sees in a year, and set the stacking or packing point from that result.

Does existing silicone dispensing equipment work for MS polymer sealant?

Existing silicone dispensing equipment often handles MS polymer sealant, because both are moisture-curing pastes pumped from cartridges, pails or drums, but the equipment must be checked for moisture entry and purged of silicone first. One MS polymer data sheet lists application from cartridges, foil cartridges, and from hobbocks or drums with high pressure pumps with follower plates (Henkel, 2021). An MS polymer maker states that the polymer cures only when both catalyst and water are present, and that "moisture should be managed properly to avoid curing in the cartridge, bucket, or other packaging material" (Kaneka Belgium, 2026).

On a line this means four checks. Pump seals, follower plates and hose ends must keep air out during breaks and weekends, since cured plugs block nozzles. The open time of a pail, from first opening to empty, should be shorter than the time the material stays usable in your climate. Storage temperature matters too: one MS polymer grade lists 10 to 25 °C storage and 12 months shelf life unopened, and notes that cold material extrudes more slowly (Henkel, 2021).

The last check is silicone carry-over. Hoses, mixers and nozzles that ran silicone hold residue, and the next section explains why that residue matters when parts are painted. Ask the equipment maker which parts can be cleaned and which must be replaced.

Why can leftover silicone on the line still cause paint defects after the switch?

Leftover silicone on the line can still cause paint defects after the switch because very small amounts of a low surface tension contaminant are enough to make wet paint pull away and form craters. BASF Coatings explains that craters are "normally caused by a low surface tension contaminate" and that "only very small amounts of the low surface tension material contamination are needed to form the crater defects" (BASF Coatings, accessed 2026). The same source lists silicones among the typical crater-causing materials and names hoses and sealing compounds as sources inside the spray booth.

Switching the sealant removes the main new source, but not the old ones. Silicone residue can stay on fixtures, gloves, tool handles, dispensing hoses and parts already sealed and waiting in stock. Each of these can carry silicone into the paint shop for weeks after the last silicone cartridge is gone. This is inference from how contamination spreads, not a measured time.

Some paint shops, especially in automotive supply chains, control this through LABS rules, which restrict paint-wetting impairment substances. The test method behind those rules is VDMA 24364:2018-05, Testing for paint wetting impairment substances (LABS-conformity), listed as current by DIN Media (VDMA 24364:2018-05). If your customer runs a LABS-controlled paint shop, ask which zone your parts enter and whether the new MS polymer grade needs a LABS conformity statement.

Which steps are hard to reverse once a line moves from silicone to MS polymer?

Three steps are hard to reverse once a line moves from silicone to MS polymer sealant: parts shipped and approved with the new material, a paint shop that has gone silicone-free, and dispensing equipment rebuilt or replaced for the new grade.

Shipped parts are the first. Once customers have parts sealed with MS polymer and have approved them, going back to silicone is a second material change with its own approval. The paint shop is the second. If the switch is part of a move to silicone-free painting, reintroducing silicone later reopens the crater risk described above (BASF Coatings, accessed 2026). Equipment is the third. Seals, hoses and nozzles changed for the new grade, and silicone stock written off, make a quick return slow.

Stress scenarios belong in the plan before the switch: a winter cure failure, a substrate that fails peel at the second supplier, a paint batch that craters. For each one, name the early signal and the point at which you would stop the rollout. The same thinking on stop signals and reversal cost, applied to formulation changes in general, is set out in What are the risks of formulation changes, and how do you decide?. Lubinpla can run this decision as a feasibility review from your own data sheets and line records, fixing the options and assumptions first and then setting out downside and stress scenarios with early signals and switch conditions in one cited document for your review.

What should you ask your MS polymer sealant supplier before the line trial?

Ask your MS polymer sealant supplier for the data that settle the six line conditions before the trial starts, because a trial that tests the wrong climate or substrate wastes a production window. Seven questions cover it.

1. What are the skin time and cure rate at 23 °C and 50% relative humidity, and at the lowest humidity in your shop?

2. Which of your substrates need a primer, and does the supplier hold peel data such as ASTM C794-18(2022) on them?

3. What service temperature range is the grade rated for, and for how long at the short-term peak?

4. Which paint systems has the grade been painted with, wet-in-wet or after cure, and with what result?

5. Does the grade carry a LABS conformity statement under VDMA 24364:2018-05, if your paint shop requires one?

6. What storage temperature, shelf life and pail open time apply, and which pump seals suit the grade?

7. Is there an ASTM C920-18(2024) classification, if your customer specification refers to one?

On the last point, ASTM C920-18(2024) covers elastomeric joint sealants for buildings, plazas and decks and classifies them by type, grade, class and use (ASTM C920-18(2024)). ASTM itself notes that not all sealants meeting the specification suit all applications and substrates, so a class match does not replace your line trial.