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Methylenediphenyl diisocyanate, known in the trade simply as MDI, holds a place both crucial and complicated in global manufacturing. Around the time my own chemical studies began, the world already leaned on MDI for its ability to build resilient yet lightweight structures. Chemists in the 19th and early 20th centuries explored all sorts of aromatic ring compounds and phosgene reactions in their pursuit of better plastics. Out of these restless decades came the discovery that reacted isocyanates produced foams and coatings with qualities essential to daily life. MDI, more robust and versatile than earlier diisocyanates, stood out because it resisted moisture and temperature changes better than alternatives. Soon after World War II, as the chemical industry raced to enable new peacetime markets, MDI’s commercial journey began in earnest. Large-scale production followed Europe’s rebuilding, then found its way across continents as economies sought affordable and reliable compounds for construction, packaging, and transportation.
Anybody who’s worked around MDI knows its visuals—a dull yellow or pale brown liquid that thickens in the cold, sometimes forming small crystals if left in storage too long or exposed to low temperatures. With a sharp, musty odor that lingers in industrial settings, MDI’s high reactivity sets it apart. The isocyanate groups snap onto polyols and water, making these crosslinked chains that resist compression and sagging. MDI boils at a high temperature, demands that storage tanks stay dry, and attacks some rubbers and even metals over time. The chemistry behind this material often leads to problems when small leaks go undetected in plant equipment or when workers grow complacent about personal protective equipment. Its reactivity forms the backbone of its usefulness—making the incredibly strong adhesive bonds for wood panels or creating insulating foams that outperform older materials by a mile.
Labels on MDI drums reveal more than just a name. Purity matters. I’ve seen formulations swing on fractions of a percent, with excess acidity or hydrolysis shrinking foam yields or leaving coatings tacky and weak. Technical documents spell out viscosity, isomer ratios, and color index, all helping buyers judge whether a batch is suited more to rigid insulation or flexible seating. In my years working on polymer development, I watched manufacturers fixate on the amount of 4,4'-MDI, often chasing specific ratios to hit the sweet spot for their equipment. Out in the plant, a worker might care more about the UN number and warning icons. The difference between MDI and its rougher cousin TDI (toluene diisocyanate) matters in practice; MDI creates less vapor and poses lower risk of acute exposure under most conditions, although chronic hazards do not shrink away just because the chemical changes name. Fact remains: it’s crucial to understand both what’s in the barrel and what job it needs to do.
To prepare MDI, chemical plants run a reaction between aniline and formaldehyde in the presence of hydrochloric acid, building up crude methylenedianiline (MDA). Later, this product reacts with phosgene—a toxic and unforgiving gas—to yield crude MDI mixtures. Years ago, I shadowed a process engineer through the maze of reactors and towers at a polyurethane plant. The mix of automation and intense operator attention surprised me. Dust, leaks, and changes in temperature all meant dangerous phosgene could escape unless pumps and seals stayed in perfect shape. Later purification steps separate MDI into variants—pure, modified, and polymeric types. Handling these intermediates takes serious preparation; small lapses lead to substantial risks. Above all, those steps mean the name “MDI” includes a range of related compounds, some excellent for bulk foam, some destined for adhesives or coatings.
In daily use, MDI does more than react just once. It opens doors to modification. React with polyether or polyester polyols, and the world gains flexible or rigid polyurethane foams. Add water in controlled amounts, and the isocyanate groups release carbon dioxide, inflating mixtures into lightweight panels or cushions. In R&D settings, chemists often tweak MDI for custom jobs—adding minor substituents or tailoring chain lengths to meet thermal, mechanical, or fire-resistance benchmarks. Early in my career, I learned the importance of keeping these changes clearly documented. Lab blends that failed last month sometimes led to the best products after one small molecular nudge. Still, every new modification forced us to return to toxicity data, workplace handling, and disposal methods; a small tweak in chemistry could mean big shifts in hazard controls or downstream process needs.
Over the years, I’ve heard MDI called too many names to count—MDI, methylene diphenyl diisocyanate, 4,4'-diisocyanato-diphenylmethane, and in some catalogs, crude MDI or polymeric MDI. Industry professionals know that abbreviations and synonym-laden labels confuse new hires and emergency responders alike. The product’s registry numbers hint at the family of chemicals covered, and industrial trade sheets often stick to “MDI” for brevity, but legal documents demand longer, more formal names. This naming tangle, while tedious, underlines a challenge: communication between manufacturers, regulators, and users has to stay clear, especially when hazards and handling matter more than sales pitches.
Out in the workplace, safety isn’t just regulatory paperwork—failures carry personal consequences. Anyone handling MDI gets a crash course in gloves, goggles, and ventilation. A minor spill on exposed skin often means a trip to occupational health. Isocyanates like MDI rank among the leading causes of occupational asthma in manufacturing. I’ve watched colleagues get benched with persistent coughs, only to find isocyanate sensitization ends careers in foam fabrication. Regulations demand monitoring of indoor air and periodic health checks, but compliance sometimes falls to workers speaking up when they smell trouble. Training, solid engineering controls, and equipment maintenance protect not just the workers but also plant neighbors downwind. Community concerns around MDI transport and storage push companies to invest in containment, leak detection, and rapid response, learning from hard-earned lessons when disasters make headlines.
Few materials cut across daily life like MDI. Most people don’t realize they lean on this chemical every time they open the refrigerator, drive a modern car, or lean back in a foam-cushioned seat. Rigid polyurethane foam insulates homes, fridges, and pipes. Flexible foams appear in bedding and furniture. Adhesives bind wood products in construction and cabinetry, offering strength at a fraction of older solvent content. As someone who has tested building materials after fire exposure, I see how fire-resistant MDI-based foams changed codes and saved lives. Industrial coatings protect bridges, pipelines, and highways thanks to MDI’s weather resistance. Even athletic gear—helmets, artificial running tracks—owes some toughness to MDI chemistry. Supply and demand ripples reach all over, with regional shortages turning into construction delays, car part bottlenecks, and cost spikes across industries.
Research into MDI keeps moving, powered by demand for lighter, safer, and cleaner materials. Chemists keep hunting for gentler substitutes for phosgene in production, but none deliver the same scale and yield yet. Life cycle assessments examine how manufacturing, use, and end-of-life disposal impact the environment. I’ve witnessed university labs focusing on bio-based polyols or innovative recycling strategies to extend polyurethane lifespans, although commercial rollout remains slow. Pressure from green building standards now pushes the industry to account for every kilogram of MDI produced and used. Time and again, collaboration between academic scientists and large chemical firms brings safer handling guidelines, new detection technologies, and richer toxicological data. Grants for studying the fate of isocyanates in water and soil attract more researchers with each passing year, keeping safety and sustainability in focus.
Toxicity headlines around MDI follow a pattern I’ve seen before: near-misses, overlooked exposures, and late-breaking studies on long-term health. Once inhaled or splashed, MDI can spark intense allergic reactions or chronic respiratory trouble. Medical journals list scores of occupational illnesses tied directly to short-shrifted precautions. In environmental incidents, MDI reaches waterways, posing risks to fish and drinking supplies if not managed well. Modern efforts at risk management grow more effective—air handling, containment, and worker health surveillance do give results. Still, questions about biodegradability and breakdown products keep popping up. While MDI breaks down into less active fragments after some time and exposure, residues linger in landfill sites or fire debris, so companies must invest in better waste management and emergency planning to reduce lasting problems.
The future for MDI looks both crowded and contested. Growth in insulation, automotive, and construction markets drives up volume, but safety and green chemistry pressures apply brake and accelerator at the same time. More companies experiment with plant-based intermediates to reduce fossil dependence. Regulators look harder at process emissions and accidental releases, tightening rules every year. In research meetings I’ve attended, leaders debate every angle—cost, reliability, worker safety, and carbon footprint. The biggest wins may yet come from digital plant monitoring or automated air sampling. Innovations in process safety and smarter control systems promise another layer of protection, making workplaces less hazardous. Meanwhile, pressure to make polyurethane foams more recyclable keeps attracting chemists worldwide. The real challenge comes from balancing economic growth, human health, and environmental limits—a mix that demands not just new chemistry, but constant attention and responsible stewardship at every stage of production and application.
Walk into almost any modern building, and you’re surrounded by the results of methylenediphenyl diisocyanate, often called MDI. This chemical gives life to much of the insulation wedged inside walls or covering the roofs above our heads. Experience tells me that opening up a wall in an old house reveals fluffy foam or rigid boards keeping the cold out. MDI built those — not because it’s a niche product, but because it forms the backbone for polyurethanes. Polyurethanes lead the charge for durable, energy-saving insulation that helps slash utility bills and keeps homes comfortable in blazing summers or icy winters.
Most folks don’t think about the features hidden inside their couch cushions or their refrigerator doors. Yet, these everyday comforts owe a debt to MDI. Polyurethane foams made from this substance give furniture their springy resilience. Old chairs used to lose their shape and comfort after a few years. My time working with furniture restoration taught me newer foam cushions simply outlast their older relatives. In the appliance world, energy-efficient refrigerators use MDI-based rigid foam to trap cold inside and keep food fresh for long stretches. Consumer Reports suggest insulation like this cuts annual fridge energy use by more than half compared to pre-2000 models.
Recalling years spent tinkering with cars, many modern vehicles rely on lightweight materials to boost fuel economy. MDI-driven polyurethane foams fill gaps and shape panels for car seats, dashboards, headrests, and even engine bays. These foams help absorb shocks, keep the ride quieter, and offer real protection during accidents. Industry data from the European Automobile Manufacturers’ Association point to thousands of tons of MDI-based foams going into cars each year. They make vehicles lighter, safer, and more comfortable to drive over city potholes or down the open highway.
Like many useful industrial chemicals, MDI comes with trade-offs. Anyone who has worked in a factory setting knows the pungent odor of isocyanates like MDI or remembers safety training drills to keep the air clean and protect skin and eyes. Inhalation of MDI can cause asthma or other respiratory troubles, especially during mixing and application. The American Conference of Governmental Industrial Hygienists sets strict limits on airborne exposure, but small contractors and DIY enthusiasts sometimes miss the message. I’ve seen cases where better ventilation, tight-fitting masks, and nitrile gloves made all the difference in avoiding long-term health issues.
While there’s no denying the pivotal role MDI plays, the future asks for safer, greener options. Some forward-thinking manufacturers experiment with plant-based alternatives and closed-loop recycling for polyurethane products. Teaming up with occupational safety experts, businesses can push for smarter engineering controls and training so workers and hobbyists stay out of harm’s way. Regulatory agencies play a part, yes — but sharing practical knowledge in shops and on job sites shifts the needle most. By lifting the standard for handling MDI, communities protect both their workers and their environment, all while keeping the benefits of insulation, comfort, and innovation at their fingertips.
MDI, short for methylene diphenyl diisocyanate, shows up in a surprising number of everyday products. Foam insulation, adhesives, paints, and even the spongy soles of running shoes rely on it for strength and flexibility. MDI does its job well, but it demands respect during handling. If you breathe MDI vapors or get skin exposure, your body will not thank you. The risks are real: asthma, chemical burns, and long-term respiratory issues. Over years of working on construction sites and refurbishing old furniture, I’ve seen too many people shrug off protective equipment until something goes sideways. No one forgets their first “MDI incident.”
MDI doesn’t need huge volumes to cause problems. The stuff can get in through lungs or even skin, starting with irritation and—if exposure continues—progressing to severe breathing trouble. The National Institute for Occupational Safety and Health (NIOSH) draws a clear line: exposure should not top 0.05 mg/m3 in air over 8 hours. Folks who don’t take symptoms seriously sometimes push through a headache or itchy hands, assuming it’s just a bad day at work. Later, when a small problem grows into an emergency, those minutes without a mask or gloves suddenly matter a lot.
Personal protective equipment is not optional. A proper half-face respirator with organic vapor cartridges keeps lungs clear of danger. Latex gloves won’t cut it against MDI—only nitrile, butyl, or other chemical-resistant gloves give a real barrier. Protective eyewear keeps splashes out, and coveralls or long sleeves block skin contact. Even those who have handled harsh chemicals for decades get lax and take risks, maybe working late to finish a project and ditching uncomfortable gear. But once someone gets sensitized to MDI, even a small whiff can trigger a severe reaction every time.
Ventilation stands as the second line of defense. Not every shop or jobsite has high-end fume hoods, but even exhaust fans and open windows make a big difference. Working outdoors beats an enclosed garage for air movement. I’ve dragged workbenches out to driveways for this reason, and it always pays off. Clean-up matters, too. Spills, drips, or even dust from sanding MDI-based products can linger if swept aside carelessly. Absorbent pads and dedicated disposable towels mean less risk, with proper disposal in sealed, labeled containers.
Safety rules mean nothing unless crews get trained up and stay alert. New workers need hands-on demonstration—donning masks, finding glove sizes, understanding symptoms. Refresher courses help old pros remember why shortcuts hurt. The Occupational Safety and Health Administration (OSHA) has published clear rules on MDI, but real safety sticks when every team member watches out for each other. I remember a coworker who always double-checked his neighbors before mixing batches. A little peer pressure kept everyone honest, and nobody complained after seeing the lasting health of the crew.
Safer MDI alternatives exist, though few match its performance and cost. Where possible, swapping to less hazardous ingredients makes sense, but most trades must manage the risk, not dodge it completely. At the end of the day, health matters more than speed or savings. Any manager or lone craftsman willing to invest in quality protection, good habits, and the occasional deep clean will almost always come out ahead.
MDI, or methylene diphenyl diisocyanate, plays a big role in making resins, adhesives, and foams. It’s useful, but this chemical packs real danger if treated like garden-variety supplies. It sensitizes people quickly. Breathe its vapors or let liquid touch skin, and you remember the burning, the chest tightness, the headaches. Some folks never stop reacting after a bad exposure. More than a few old-timers left industrial work for good after MDI accidents. These stories leave a mark on anyone handling the stuff.
Let MDI freeze, and good luck getting it flowing again. Producers recommend keeping it between 18°C and 30°C, which means above freezing but not hot. Heat makes MDI react, releasing toxic vapors and pushing up container pressure. I’ve watched drums sweat and bulge on hot concrete, one summer afternoon—crews grabbing fans, scrambling to move them out of the sun. Cold, on the other hand, thickens MDI, forming crystals. Thawing it too fast degrades the product or splits the drum. Smart operators keep MDI indoors or under well-ventilated canopies, away from sunlight or heaters.
People new to MDI sometimes ask if a spare plastic barrel will do. Not even close. MDI reacts slowly with most plastics and rubber, which leads to leaks after a few months. Steel drums with proper linings—usually epoxy-phenolic—handle MDI best. Railcars, tank trucks, and even intermediate bulk containers use robust steel matched with tight seals. Each sealing point uses gaskets made from PTFE or similar tough elastomers, since the wrong gasket will fail and leak dangerous fumes.
MDI reacts fiercely with water—forming carbon dioxide, pressure that warps drums, and thick urea side-products that clog equipment. Simple air can bring enough moisture to create problems. Shipping and storage always involve dry nitrogen blanketing above the liquid. This layer of gas, kept under slight pressure, prevents water vapor from sneaking in. By the time you smell amines or see yellow sludge, it’s far too late.
You spot the difference right away between sloppy and careful outfits. Clean, readable labels matter. Every container lists the right hazard information and handling instructions. Emergency gear—showers, eye wash stations—sits within twenty steps. Caked drums and smudged instructions lead to confusion, which leads to hurt workers.
Drivers need hazmat training before taking MDI on the road. Sudden stops, rollovers, punctured tanks—none of these leave room for guesswork. Inspections before filling, secure fastenings, checks on pressure relief valves, and paperwork in plain reach all cut risk. I’ve seen shipments stuck at borders after handlers missed document updates, causing delays and cost. No matter how urgent, nobody wins if a shortcut leads to a spill or exposure.
Safety around MDI doesn’t fall on one person or even one department. Health and safety officers, warehouse workers, drivers—all rely on each other. Sharing stories, enforcing rules, and refusing to compromise keeps people healthy and lets businesses keep moving.
Mistakes can linger in air, lungs, and work histories. The only real solution is a mix of respect, careful storage conditions, dry environments, the right containers, and clear teamwork. Good practice today avoids the severe costs of carelessness tomorrow.
MDI, short for methylene diphenyl diisocyanate, plays a leading role in polyurethane foam production—think furniture, insulation, shoes, and even car seats. As someone who’s worked in manufacturing, I’ve learned that small oversights turn into big headaches. Shelf life tells us how long MDI keeps its original properties, which means it still reacts properly, blends smoothly, and sticks to quality standards. Left on the shelf beyond its time, MDI can turn from a valuable tool to wasted money and project setbacks.
Straight from the factory, pure MDI usually lasts about one year under tightly controlled conditions. I’ve seen warehouse managers struggle when temperatures swung too high or shelves grew too crowded. Factory-packed, unopened drums kept cool (15°C–25°C), dry, and away from direct sunlight give MDI its best shot at a full shelf life. Let that slide, and its lifespan shortens rapidly.
Moisture is enemy number one. Isocyanates crave water; even a small leak could start a reaction, forming urea crystals and thickening the liquid. You might open a drum and find gunk at the bottom or a weird viscosity that wrecks dosing accuracy. Once, a client ignored a leaky warehouse roof; quality checks flagged the batch as unusable, forcing a costly disposal and cleanup. So, sight and smell matter—a sharp nose or a careful eye might spot trouble before numbers do.
Using MDI that’s past its prime can throw off every step of a production run. Properties like reactivity, color, and flow shift from the norm. In finished products, that means foam that slumps, cracks, or absorbs more water. I’ve seen discarded shipping containers full of faulty panels—all because someone gambled on expired MDI to keep up with a deadline. The losses stretch beyond production, into energy bills, customer complaints, and recalls.
Keeping track of inventory dates saves time and money. Digital inventory tools, barcodes, or even old-fashioned color-coded tags help spot aging stock. Training warehouse staff on the signs of contamination or crystallization puts another set of eyes on quality control. Frequent stock rotation—first in, first out—does a lot more than fancy automation, especially in smaller outfits with tight staffing.
Suppliers can also lend a hand: up-to-date safety data sheets, warning labels, regular lot testing, and reminders about expiration dates should be standard. In my experience, the better the relationship with your chemical supplier, the easier it is to replace aging stock before it turns into waste.
Some companies wait for trouble before changing habits. A more proactive approach—regular shelf checks, solid supplier communication, and mandatory training—keeps teams prepared. Government guidelines from agencies like OSHA or the EPA give good baseline rules, but nothing replaces accountability on the shop floor. I encourage plant managers to make shelf life reviews a monthly habit, not a yearly afterthought.
MDI is powerful stuff, but it doesn’t last forever. Smart practices give every batch its best shot and keep headaches out of the shop. Better monitoring and open conversations around shelf life will keep MDI doing its job well—and keep teams out of trouble.
Anyone who’s worked in a foam plant or built insulation panels knows the smell of MDI—methylene diphenyl diisocyanate. It’s scratchy stuff, not something I’d want near my garden tools, and definitely not something to toss in the regular trash. MDI isn’t just hard on your lungs; it’s no friend to rivers or groundwater. The real challenge kicks in after you finish using it: what do you do with leftover drums or scraps?
Let’s start with the basics. MDI reacts fast and stays toxic. People might think letting it harden and throwing it away once it’s solid is all right, but that’s a shortcut with big risks. Runoff from broken cartridges or fragments dumps dangerous chemicals into soil and water, sticking around for years. It won’t magically vanish, and you can't just hope for the best with chemicals that link to asthma, skin problems, and more severe health issues.
From my days around chemical storage and disposal, I learned that every place has strict rules for hazardous waste, but there’s more to it than checking boxes. You have to think about where every ounce ends up—not just whether you meet a company’s procedure. That’s real responsibility, since everyone downstream pays for chemicals left behind.
MDI waste must go in special, clearly labeled containers—no mistakes here. Most plants use steel drums with lined insides and sturdy, leakproof seals. Caps matter as much as the label. Keep those barrels in cool, dry corners, nowhere near sunlight or water sources, and far away from forklifts or tools that might puncture them.
Only certified waste contractors should get near this waste once it leaves the shop floor. These outfits know local and national rules inside and out. They ship MDI for incineration at extremely high temperatures, which breaks down the nastiest bits into safer, manageable leftovers. This isn’t the job for municipal dumps or casual recyclers. Local governments often keep a list of approved handlers, and big manufacturers usually work directly with these disposal companies. Skipping this step can land people and companies with big fines—or worse, contaminated groundwater their neighbors drink.
Chemistry professors and process engineers have been experimenting with ways to neutralize leftover MDI. Some plants treat small spills with specific deactivators—usually chemicals that break down MDI into less harmful compounds. In practice, most shops don’t try to run their own neutralization systems. The risk is too high; people handling these reactions need training, and mistakes aren’t easy to clean up. But these ideas point to a larger hope: as technology moves forward, we’ll see more tools and kits on the market that make small-volume deactivation safer and cheaper for small operators.
The only real answer lies in treating leftover chemicals with as much respect as the finished product. Don’t shortcut and don’t rely on guesswork. Employees must get training about how MDI impacts the body and the land. Shop managers should develop clear routines, check up on suppliers, and keep records. If something spills or disappears, reporting matters—and covering up risks harm for many others. An honest look around every workspace and an unwillingness to cut corners go much further than any pile of manuals ever could.
| Names | |
| Preferred IUPAC name | 1,1'-Methylenebis(4-isocyanatobenzene) |
| Other names |
4,4’-Diphenylmethane diisocyanate Diphenylmethane-4,4’-diisocyanate MDI Methylenebis(phenyl isocyanate) Bis(4-isocyanatophenyl)methane |
| Pronunciation | /ˌmɛθ.ɪˌliːn.daɪˈfɛn.əl daɪ.aɪ.səˈsaɪ.əˌneɪt/ |
| Identifiers | |
| CAS Number | 101-68-8 |
| Beilstein Reference | 1461122 |
| ChEBI | CHEBI:53773 |
| ChEMBL | CHEMBL1200901 |
| ChemSpider | 84969 |
| DrugBank | DB14055 |
| ECHA InfoCard | 03ecd471-254c-47e4-8ca0-702a9bfa62a0 |
| EC Number | 615-005-00-9 |
| Gmelin Reference | 204568 |
| KEGG | C06435 |
| MeSH | D003639 |
| PubChem CID | 8613 |
| RTECS number | NZ4370000 |
| UNII | K0IUNM943E |
| UN number | UN2077 |
| CompTox Dashboard (EPA) | DTXSID2020737 |
| Properties | |
| Chemical formula | C15H10N2O2 |
| Molar mass | 250.25 g/mol |
| Appearance | Light yellow to pale brown liquid or solid |
| Odor | Musty odor |
| Density | 1.23 g/cm³ |
| Solubility in water | Insoluble |
| log P | 4.51 |
| Vapor pressure | 0.0002 mmHg (25 °C) |
| Acidity (pKa) | 12.3 |
| Basicity (pKb) | pKb: 7.92 |
| Magnetic susceptibility (χ) | -7.1 × 10⁻⁶ cm³/mol |
| Refractive index (nD) | 1.574 |
| Viscosity | 50–250 mPa·s |
| Dipole moment | 3.96 D |
| Thermochemistry | |
| Std molar entropy (S⦵298) | 369.6 J·mol⁻¹·K⁻¹ |
| Std enthalpy of formation (ΔfH⦵298) | −104.3 kJ/mol |
| Std enthalpy of combustion (ΔcH⦵298) | -5712 kJ/mol |
| Pharmacology | |
| ATC code | V04CX12 |
| Hazards | |
| Main hazards | Harmful if inhaled, causes skin and eye irritation, may cause allergic skin and respiratory reactions, suspected of causing cancer, may cause damage to organs through prolonged or repeated exposure. |
| GHS labelling | GHS02, GHS05, GHS07, GHS08 |
| Pictograms | GHS07, GHS08 |
| Signal word | Warning |
| Hazard statements | Hazard statements: H315, H317, H319, H332, H334, H335, H351, H373 |
| Precautionary statements | P261, P264, P271, P272, P280, P284, P302+P352, P304+P340, P305+P351+P338, P308+P313, P312, P314, P362+P364, P403+P233, P405, P501 |
| NFPA 704 (fire diamond) | 3-1-1-W |
| Flash point | 199.4 °C |
| Lethal dose or concentration | LD50 oral rat 9200 mg/kg |
| LD50 (median dose) | LD50 (median dose): >9,400 mg/kg (oral, rat) |
| NIOSH | NA0006 |
| PEL (Permissible) | 0.02 ppm |
| REL (Recommended) | 0.05 mg/m³ |
| IDLH (Immediate danger) | 75 mg/m3 |
| Related compounds | |
| Related compounds |
Toluene diisocyanate Hexamethylene diisocyanate Polymeric MDI Diphenylmethane diisocyanate Naphthalene diisocyanate |
