If you formulate polypropylene compounds, you have reached for maleic anhydride grafted polypropylene before. It is the quiet workhorse that keeps a lot of PP modification from falling apart. The catch is that “PP-g-MAH” is not a single material. Change the base polymer, the process, the melt flow, or the grafting level and you get something that behaves nothing like its cousin in the next bin.
This piece covers the five grades you will actually meet in a spec sheet: reactive extrusion, high-flow, PP wax, homopolymer, and copolymer. My goal is simple. By the end you should know which one belongs in your blend without running three failed trials first.
What PP-g-MAH is, without the textbook voice
Polypropylene is non-polar. That is useful for some things and a real headache for others, because it does not grab onto polar materials like glass fiber, nylon, or EVOH on its own.
The trick is to graft maleic anhydride (MAH) onto the PP chain. Those MAH groups are polar. They react with the amine groups sitting on glass fiber surfaces, they condense with the end groups of nylon, and they interact with other polar polymers. So the non-polar PP ends up chemically hooked to polar substrates, and two phases that wanted to separate get pulled into one stable blend.
One idea, graft a polar group onto a non-polar chain, explains why this stuff shows up in so many compatibilizing jobs.
Why two “PP-g-MAH” products can be strangers
Here is the part that bites people. Two products can both carry the PP-g-MAH label and still refuse to act alike in your recipe. Four things decide the behavior:
- The base polypropylene, whether it is homopolymer, copolymer, or a wax-grade oligomer.
- How it was made, with reactive extrusion being the common route.
- The melt flow index, basically how freely the melt moves.
- The grafting level, meaning how much MAH is actually attached, and we usually read it as a percentage.
Nudge any of those and the material slides from general-purpose workhorse to thin-wall molding specialist to natural-fiber coupling aid. Let’s go through them.
Reactive extrusion grade: the default you reach for first
This is the most straightforward version of PP-g-MAH. Inside a twin-screw extruder, maleic anhydride grafts onto the polypropylene chain with help from a peroxide initiator, all while the mix is molten and reacting.
Its real strength is grafting efficiency and cleanliness. Chemical separation plus FTIR work shows reactive-extrusion product comes out clean, with almost no free maleic anhydride floating around unreacted. Like plain PP, the grafted material is a non-Newtonian pseudoplastic fluid, which means it molds without fuss.
A few numbers worth keeping handy:
- Grafting level usually lands between 0.8% and 1.6%.
- The newer odorless, high-grafting variants keep the same thermal stability and compatibilizing punch while dropping the smell and raising the graft. That is the direction the more environmentally conscious compatibilizers are heading.
- Typical addition sits at 3% to 8% of the formulation.
It fits almost any PP modification job: glass-fiber-reinforced PP, mineral-filled PP, PP/PA alloys, PP/EVOH blends, and wood-plastic composites. The limitation is honest. Reactive extrusion grade only builds the bridge. Its base is ordinary polypropylene, not an elastomer, so it compatibilizes and stops there. No extra toughening. If your system needs impact, grab something else.
Use it for glass-fiber-reinforced PP, mineral-filled PP, PP/PA alloys, PP/EVOH blends, and wood-plastic composites. It is the most versatile of the five, and you add 3% to 8%.
High-flow grade: for thin walls and parts that ought to be bigger
High-flow grade starts where reactive extrusion leaves off and pushes the melt flow index higher through formulation and process changes. Plain PP-g-MAH tends to sit at an MFI of 30 to 70. High-flow grades climb to 80, 110, sometimes beyond.
The job is narrow but important: thin-wall parts and large molded articles need high melt flow. Thin walls will not fill the mold if the melt crawls. Big parts force you into ugly injection pressures and long cycles when flow is weak. High-flow PP-g-MAH gives you the compatibilizing effect without pulling the system’s flow down, and sometimes it lifts the overall flow.
The data is not modest. In one PP plus 30% glass fiber system, dropping in 3% of a high-flow compatibilizer (MFI 80 to 110, grafting at or above 0.8%) took tensile strength from 45 MPa to 85 MPa and notched Izod impact from 60 J/m to 120 J/m.
The cost of that flow is molecular weight. Higher MFI means lower molecular weight, and that caps both the compatibilizing efficiency and the ceiling on mechanical properties. If you are pushing a system to its mechanical limit, the standard reactive extrusion grade is the safer bet.
Use it for thin-wall injection-molded PP, large PP parts, and filled systems that demand flow. Add around 3%.
PP wax (high acid value): lowest molecular weight, highest polarity
PP-g-MAH wax is not the same beast as the first two. It is maleic-anhydride-grafted polypropylene wax, and its molecular weight is in the low thousands. That is roughly an order of magnitude below standard PP-g-MAH, which runs in the tens of thousands to past one hundred thousand.
Low molecular weight buys you very low melt viscosity, great flow, and strong penetration. In hot-melt adhesives it soaks into the substrate surface and forms tight chemical bonds. In glass-fiber and natural-fiber-reinforced PP it wets the fiber and lets the coupling agent do its full job.
The second spec that matters is acid value. Clariant’s Licocene PP MA 6452 runs 37 to 45 mg KOH/g. Honeywell’s A-C 597P gets close to 90. Higher acid value means stronger polarity and faster reaction with polar materials.
Where it gets used diverges from standard PP-g-MAH. You see it more in hot-melt adhesives, floor waxes, coatings, and inks than in injection molding. Inside plastics work, it mainly serves as a coupling agent for glass and natural fiber reinforcement and as a dispersant in heavily filled systems.
The weakness is plain: molecular weight this low will not carry structural compatibilization. It tightens the interface but adds little strength of its own. For structural part modification, standard PP-g-MAH is the better tool.
Use it as a coupling agent in glass- and natural-fiber-reinforced PP, a dispersant in heavily filled systems, and an adhesion promoter in hot-melt adhesives. It lives more outside injection molding than in it.
Homopolymer PP-g-MAH (high MFI): stiffest, best matched to homopolymer PP
This grade uses homopolymer polypropylene as the base, and homopolymer PP has a regular chain, high crystallinity, and high stiffness.
Its edge is being the same material as the homopolymer PP matrix. When you reinforce or fill homopolymer PP, homopolymer PP-g-MAH gives the tightest matrix match and the most direct compatibilizing effect. High-MFI versions add flow on top of that stiffness. One grade reaches MFI 35 to 70, while a high-flow variant reaches 80 to 110.
The downside is toughness. Homopolymer PP is rigid but brittle next to its copolymer cousin, so the compatibilizer adds coupling and nothing for impact. For systems that also need to absorb a hit, copolymer PP-g-MAH or an elastomer-grafted product fits better.
Use it for glass-fiber-reinforced and mineral-filled homopolymer PP, and for PP modification where stiffness and dimensional stability sit at the top of the list.
Copolymer PP-g-MAH (high impact): toughest, matched to impact systems
Copolymer PP-g-MAH uses copolymer polypropylene, usually the block copolymer kind, as the base. The ethylene comonomer gives the chain more give than homopolymer PP has.
Its biggest draw is that it toughens while it compatibilizes. Work on grafting shows MAH attaches to copolymer PP at a higher level than to homopolymer PP. In high-impact grades, adding 10% to 18% lifts notched Izod impact by 130% to 160%, holds room-temperature impact above 55 kJ/m², and still clears 20 kJ/m² at minus 30 °C.
It is the obvious pick for PP systems that must take a hit: automotive bumpers, instrument panels, appliance housings, parts that need strength and flexibility at once. In PP/EPDM elastomer blends, copolymer PP-g-MAH and EPDM also toughen together, which is a nice bonus.
The trade is rigidity. It will not match homopolymer PP-g-MAH on stiffness, so for filled systems where rigidity rules, the homopolymer route wins.
Use it for glass-fiber-reinforced copolymer PP, PP/EPDM elastomer blends, automotive parts, and appliance housings. Add 3% to 18%.
Picking between the five without guessing
Short version, in the order I would reach for them:
- Need maximum versatility and no special flow or toughness demand? Reactive extrusion grade. Most mature process, widest range, most field history. Add 3% to 8%.
- Thin-wall molding, big parts, strict flow needs? High-flow grade. MFI 80 to 110, compatibilizes without hurting flow. Add about 3%.
- Coupling glass or natural fiber, dispersing a heavily filled system, hot-melt adhesion? PP-g-MAH wax. Lowest molecular weight, highest acid value, strongest penetration, but not for structural compatibilization.
- Homopolymer PP reinforcement or filling, high stiffness and dimensional stability? Homopolymer PP-g-MAH (high MFI). Best matrix match, stiffest, weaker on toughness.
- Copolymer PP, PP/EPDM blends, automotive and appliance parts that need impact? Copolymer PP-g-MAH (high impact). Toughens while compatibilizing and can lift impact past 130%.
Things I wish someone had told me before a batch went wrong
A few habits that have saved me from scrapped runs:
- Grafting level drives the compatibilizing result. Too low and the interfacial reaction never finishes, so the benefit never shows. Ask your supplier for the grafting range. Reactive extrusion grade runs 0.8% to 1.6%; high-acid-value wax can reach about 7%.
- More is not automatically better. Every system has a sweet spot, and push past it and properties sag. Start low, find the peak, stop.
- Match the base polymer first. Homopolymer PP systems want homopolymer PP-g-MAH. Copolymer PP systems want copolymer PP-g-MAH. A mismatch quietly shaves the effect down.
- Do not swap high-flow and standard grades as if they were the same thing. High-flow has higher MFI and lower molecular weight, so its compatibilizing ceiling and mechanical ceiling sit lower. For peak mechanical performance, standard grade is the steadier choice.
- Wax doses by its own logic. Wax is low molecular weight and efficient, so you normally need less than standard PP-g-MAH. Do not copy the standard grade’s dosing habit. Start low and build up.
Where this leaves you
PP-g-MAH looks like one ingredient on the order form and behaves like a family on the lab bench. Reactive extrusion grade is your generalist. High-flow grade is for moldability. Wax is for coupling and dispersion. Homopolymer grade is for stiffness. Copolymer grade is for impact. Choose by what your matrix needs most, match the base polymer, watch the grafting level, and dose by the grade’s own rules. Get those right and the compatibilizer does exactly the job you brought it in for.