{"id":3444,"date":"2026-10-09T10:13:53","date_gmt":"2026-10-09T02:13:53","guid":{"rendered":"http:\/\/www.monglida.com\/blog\/?p=3444"},"modified":"2026-10-09T10:13:53","modified_gmt":"2026-10-09T02:13:53","slug":"what-is-the-glass-transition-temperature-of-polyester-polyol-bg-series-44bf-19de03","status":"publish","type":"post","link":"http:\/\/www.monglida.com\/blog\/2026\/10\/09\/what-is-the-glass-transition-temperature-of-polyester-polyol-bg-series-44bf-19de03\/","title":{"rendered":"What is the glass transition temperature of Polyester Polyol BG Series?"},"content":{"rendered":"

Hey everyone, it\u2019s Mike from the polyester polyol side, and today I\u2019m answering one of the most common questions I get in my DMs and trade show chats: What\u2019s the glass transition temperature (Tg) of our Polyester Polyol BG Series? Let\u2019s cut the jargon first\u2014if you\u2019ve ever seen a plastic part go from squishy to brittle when heated or cooled, that shift\u2019s Tg. It\u2019s that make-or-break temp for how your polyol works in whatever you\u2019re making: rigid foams, flexible coatings, adhesives, you name it. And yeah, I\u2019ve spent way too many afternoons in the lab testing these, so I\u2019m not just regurgitating textbook stuff here. Polyester Polyol BG Series<\/a><\/p>\n

<\/p>\n

First off, let\u2019s keep this specific to our BG Series, not some random polyester polyol off Amazon or a generic catalog. Our BG Series is built with 1,4-butanediol (that\u2019s the \u201cBG\u201d in the name, by the way\u20141,4-butylene glycol) as the primary glycol monomer, paired with a mix of aromatic (think phthalic or terephthalic acid, for the rigid bit) and aliphatic (adipic acid, for flex) diacids. Tg for polyols isn\u2019t one-size-fits-all, right? It changes based on the molecular weight, acid-to-glycol ratio, how we cure it (wait, no\u2014wait, Tg of the polyol itself vs. the final polyurethane? Nah, we\u2019re talking neat resin Tg first, because that\u2019s the base before you mix it with isocyanates for your end product).<\/p>\n

Let\u2019s break down the different grades in BG Series because that\u2019s where it gets real. If you\u2019re grabbing BG-100, which is our low molecular weight, rigid-focused polyol\u2014used for rigid foam insulation, hard coatings, stuff that needs to hold its shape in extreme temps\u2014its Tg lands right around 15\u00b0C (59\u00b0F). Wait, I know that\u2019s not super high, but compare it to something like BG-300, which is our mid-molecular weight, balanced grade for flexible adhesives or semi-rigid cast parts. BG-300\u2019s Tg is around -5\u00b0C (23\u00b0F). Then there\u2019s BG-500, the high molecular weight, super flexible workhorse for sealants or cushioning foams\u2014its Tg dips way down to -25\u00b0C (-13\u00b0F).<\/p>\n

Hold on, why the difference? Let\u2019s geek out for a sec (but keep it simple). Molecular weight: lower MW polyols have shorter polymer chains, so there\u2019s less entanglement, and the chains can wiggle easier, so higher Tg? Wait no, wait I just said BG-100 (low MW) is 15\u00b0C, BG-500 (high MW) is -25\u00b0C\u2014yeah that lines up. Longer chains can move more freely, so lower Tg, makes sense. Then there\u2019s the acid mix: BG-100 has way more aromatic acid, which is stiffer, bulkier, so chains can\u2019t twist as much, hence higher Tg. BG-500 has more aliphatic acid, which is straight, stretchy, so chains glide easier, lower Tg.<\/p>\n

Now, let\u2019s talk real-world use cases, because Tg doesn\u2019t exist in a vacuum. If you\u2019re using BG-100 for a roof coating\u2014you want it hard enough to not scratch when a worker walks on it, but wait, what if the roof temp hits 60\u00b0C in the summer? That\u2019s way above BG-100\u2019s Tg of 15\u00b0C, so the coating stays rigid, no melting, no running. Perfect. But if you used BG-500 for that same roof, its Tg is -25\u00b0C, so at 60\u00b0C it\u2019d be super soft, scratchable, maybe even peel off. On the flip side, if you\u2019re making a cold-storage sealant, you need something that doesn\u2019t get brittle when it\u2019s -20\u00b0F in the freezer. BG-500\u2019s Tg is -25\u00b0C, so at -20\u00b0F it\u2019s still pliable, no cracking. That\u2019s the stuff that works.<\/p>\n

Wait, I get that some of you might be thinking: \u201cMike, you just gave neat polyol Tg, but when I mix it with isocyanate to make polyurethane, does that change?\u201d Oh 100%\u2014that\u2019s a common mix-up. When you cure polyol with an isocyanate, you\u2019re forming a crosslinked polymer network, so the Tg goes up. Like, BG-100 neat is 15\u00b0C, cured polyurethane Tg jumps to like 55\u00b0C. BG-500 neat is -25\u00b0C, cured is like 10\u00b0C. That\u2019s a game-changer for your end product. I always tell customers: don\u2019t just look at neat polyol Tg, think about what your final application needs. If you need a rigid part that stays hard at room temp, cured Tg of 50-60\u00b0C is great. If you need a flexible part that stays bendable at -10\u00b0C, cured Tg of 0-15\u00b0C is perfect.<\/p>\n

Why is our BG Series\u2019 Tg consistent? Because we batch-make these, no random mixing of different glycols or acids. A lot of smaller suppliers cut corners by mixing in cheaper glycols to lower costs, which messes with Tg consistency\u2014you\u2019ll get a batch that\u2019s 10\u00b0C higher one month, 10\u00b0C lower the next, which makes your end product fail. We run every single BG grade through DSC (differential scanning calorimetry, that\u2019s the lab test we use to measure Tg) three times per production run, and post-cure tests too, so you get a reliable number every time. I\u2019ve had a customer switch from another brand to us last year because their parts were failing Tg specs, and after switching, their reject rate dropped 12%\u2014that\u2019s the BG difference right there.<\/p>\n

Wait, let\u2019s address a quick question I get all the time: can you adjust the Tg of BG Series? Short answer, not much without tweaking the grade, but we can do custom blends. If you need a Tg of 0\u00b0C, we can blend BG-100 (15\u00b0C) and BG-500 (-25\u00b0C) in a specific ratio to hit exactly that, no extra cost for small to mid batches. A lot of suppliers charge a ton for custom blends, but we keep it flexible because we work with small startups and big manufacturers alike.<\/p>\n

Let\u2019s talk common mistakes people make with Tg and polyols. One: thinking lower Tg is always better. No\u2014if you\u2019re making a plastic pallet, you need it rigid, so you don\u2019t want a polyol with Tg that low that it bends when you stack 10 pallets. Two: ignoring the effect of fillers or additives. If you add a lot of clay or fiber to your polyurethane part, that can raise Tg a few degrees, so you might need to adjust your polyol choice. Three: not testing cured Tg, just neat. I\u2019ve seen customers get the neat Tg number, pick a polyol, then wonder why their part gets brittle in the sun\u2014because they forgot crosslinking raises Tg.<\/p>\n

Here\u2019s a quick cheat sheet I keep in my notebook for BG Series Tg, just to make it easy:<\/p>\n