Many compounders produce MAH-g-PP in-house using MAH (from Sinolook) + base PP + peroxide in their own extruder, avoiding the premium of branded compatibiliser grades.

Optimal GD for most GF-PP composite applications is 0.5–0.9 wt% - balancing adhesion improvement with minimal MW degradation.

MAH-g-LLDPE (linear low-density PE) is the preferred tie-layer resin for coextruded barrier packaging because its flexibility matches the multilayer film structure. MAH-g-HDPE is used where stiffness is needed (rigid containers with nylon barrier layer).

Glass fibres in GF-PP composites are sized with a chemical surface treatment - typically an aminosilane coupling agent (e.g., 3-aminopropyltriethoxysilane, APTES). This leaves –NH₂ groups exposed on the glass fibre surface. When MAH-g-PP is present in the melt during compounding, the pendant anhydride groups react with these surface –NH₂ groups:

This covalent bond across the PP/glass interface dramatically improves load transfer between the stiff glass fibres and the PP matrix.

• Front-end modules: 30–40% GF-PP replaces steel stampings; weight saving 40–60%; MAH-g-PP at 3–5 wt% loading
• Bumper beams / energy absorbers: 30% GF-PP; stiffness and toughness balanced by MAH-g-PP level
• Door modules and underbody shields: 20–30% GF-PP; cost-performance optimisation
• Instrument panel carriers: High MAH-g-PP loading (4–5%) for maximum stiffness in thin-wall parts
• Engine compartment components: Heat-stabilised 30–40% GF-PP compounds; MAH-g-PP + antioxidant package

WPC decking, fencing, and automotive interior panels use PP or HDPE as the polymer matrix with 50–70% wood flour or natural fibre (bamboo, flax, jute). MAH-g-PP or MAH-g-PE at 1–4 wt% loading compatibilises the hydrophilic wood surface (–OH groups) with the hydrophobic polyolefin. The anhydride groups esterify the wood cellulose surface –OH groups, creating covalent bonds across the interface. Result: flexural modulus increases 40–80%; water uptake decreases 30–50%; dimensional stability improves significantly.

Natural fibre-reinforced PP (flax-PP, hemp-PP, sisal-PP) for automotive door panels and package trays uses MAH-g-PP as the coupling agent - same anhydride-to-cellulose –OH bonding mechanism as WPC, but applied to longer fibres. Natural fibre composites with MAH-g-PP compatibilisation achieve tensile strengths of 60–90 MPa at 30% fibre loading - competitive with short GF-PP on a specific strength basis, with lower density and better end-of-life recyclability.

MAH-g-PP improves bonding between PP and inorganic fillers whose surfaces carry –OH groups: calcium carbonate (CaCO₃, talc, kaolin), magnesium hydroxide (flame retardant), and barium sulphate. In talc-filled PP (5–20% talc for stiffness and dimensional stability), 1–2% MAH-g-PP increases flexural modulus and reduces brittleness by improving the polymer-filler interface. Magnesium hydroxide (MDH) flame-retardant PP compounds require MAH-g-PP at 2–4% to maintain acceptable impact strength at the 50–60% MDH loadings needed for V-0 rating.

Q1: What is maleic anhydride grafted polypropylene (MAH-g-PP) and what is it used for?

Maleic anhydride grafted polypropylene (MAH-g-PP, also written PP-g-MAH or MAPP) is polypropylene on whose backbone pendant cyclic anhydride groups have been installed by free-radical reactive extrusion. The grafting process involves melt-mixing polypropylene pellets with maleic anhydride (0.5–2.0 wt%) and an organic peroxide initiator (0.05–0.15 wt%) in a co-rotating twin-screw extruder at 180–220°C. The peroxide generates carbon radicals on the PP backbone (by hydrogen abstraction); these radicals add across the MAH C=C double bond, installing a pendant anhydride group on the PP chain. The grafting degree (GD) - typically 0.3–1.5 wt% MAH - determines the density of anhydride functionality. MAH-g-PP is used in three main application families: (1) as a coupling agent in glass fibre–reinforced PP composites (GF-PP), where the anhydride groups bond covalently to aminosilane sizing on glass fibres, improving tensile strength by 30–50% and impact strength by 50–80%; (2) as a reactive compatibiliser in PA6/PP polymer alloys, where the anhydride reacts with PA6 chain-end amine groups to build an in-situ block copolymer at the interface, reducing PA6 domain size and improving mechanical properties; and (3) as a coupling agent in wood-plastic composites (WPC) and natural fibre composites, where the anhydride esterifies cellulose –OH groups on fibre surfaces. Most large automotive polymer compounders produce MAH-g-PP in-house using MAH (from chemical suppliers like Sinolook) in their reactive extrusion lines, rather than buying branded compatibiliser grades, significantly reducing raw material costs.

Q2: How do I measure the grafting degree of MAH-g-PP?

The grafting degree (GD) of MAH-g-PP - the weight percentage of MAH successfully grafted onto PP - is most commonly measured by FTIR spectroscopy and acid-base titration: (1) FTIR method (rapid, qualitative to semi-quantitative): Press a thin film of the MAH-g-PP sample at ~200°C. Record FTIR spectrum; look for the characteristic cyclic anhydride doublet at 1,780 and 1,850 cm⁻¹. Quantify by comparing the area ratio of the anhydride peak (1,780 cm⁻¹) to a reference PP peak (1,460 cm⁻¹ CH₂ bend or 2,722 cm⁻¹ CH stretch). Calibrate against samples of known GD from titration. This is the standard QC method in most compounding operations - quick (15 minutes), requires no solvent, and reliably distinguishes grafted anhydride from free MAH residue (which can be removed by washing with methanol); (2) Titration method (quantitative, absolute): Dissolve 1–2 g MAH-g-PP in hot xylene; add a known excess of primary amine (typically n-butylamine or aniline) to convert all anhydride groups to amide/amic acid; back-titrate the excess amine with standardised HCl to determine the total anhydride equivalents. GD (wt%) = (anhydride equivalents × 98.06 g/mol) / sample weight × 100. This is the reference method for accurate GD determination - typically run quarterly or when verifying a new batch of MAH.

Q3: Why does PP degrade (lose MW) during MAH grafting but PE does not?

The molecular weight loss during MAH grafting of PP (but not PE) is due to a β-scission side reaction that is unique to the tertiary carbon radical formed on polypropylene. In PP grafting, the peroxide generates a radical on a tertiary carbon (every other carbon in PP is a tertiary –CH– with three carbon substituents). This tertiary macroradical can either add MAH (desired) or undergo β-scission - breaking the C–C bond adjacent to the radical to form a shorter PP chain and a terminal alkene. PP's tertiary radical is intrinsically more prone to β-scission than secondary radicals because the resulting alkyl radical and alkene are both stabilised by substitution. In contrast, HDPE and LLDPE have only secondary carbon radicals (–CH₂– with two carbon substituents). Secondary radicals are much less prone to β-scission because the resulting primary radical would be higher energy. Therefore, MAH-g-HDPE and MAH-g-LLDPE retain their molecular weight much better than MAH-g-PP. In practice, MAH-g-PP has a higher melt flow index (MFI) than the base PP - typically 2–10× higher - due to β-scission reducing average chain length. This is actually beneficial in some applications (better flow for thin-wall injection moulding of GF-PP parts) but detrimental in others (impact resistance suffers at very high MFI). The β-scission rate can be minimised by: using higher MAH concentration (competes with scission for the radical), lower peroxide level, lower temperature profile, and shorter extruder residence time.

Q4: What is the typical MAH loading used in glass fibre–reinforced PP and PA6/PP blends?

For glass fibre-reinforced PP (GF-PP) composites, the MAH-g-PP compatibiliser is typically used at 2–5 wt% of the total compound weight. For a compound containing 30% glass fibre and 70% polymer matrix, a typical formulation is: 65–67% base PP, 3–5% MAH-g-PP (1.0% GD), 30% glass fibre (chopped strand, 10 μm diameter, aminosilane-sized). Higher MAH-g-PP loading increases stiffness and strength up to an optimum (~4–5%), beyond which excess MAH-g-PP acts as a diluent with no additional compatibilisation benefit. For PA6/PP alloys, the MAH-g-PP level depends on the PA6 content and target morphology: at 20–30% PA6 in PP, typically 5–8 wt% MAH-g-PP (0.7–1.0% GD) is used to fully compatibilise the PA6 droplet phase; the MAH-g-PP loading is approximately stoichiometric with the PA6 amine end group concentration - if PA6 has 40 μeq/g amine end groups and is used at 25% in the blend, approximately 10 μeq/g anhydride equivalents must be provided by the MAH-g-PP to fully react with all PA6 chain ends. This stoichiometric approach is the most rigorous method for determining the required MAH-g-PP loading.

Q5: Can I produce MAH-g-PP in-house and what MAH grade should I use?

Yes - producing MAH-g-PP in-house in a reactive extrusion line (twin-screw extruder, L/D ≥36) is technically straightforward and economically attractive for compounders with sufficient volume (typically ≥500 MT/year of compound requiring compatibiliser). The process: dry-blend PP pellets with MAH flakes (0.5–1.5 wt%) and peroxide (0.05–0.10 wt% DCP or DHBP) in a tumble blender; feed the blend to the twin-screw extruder at 180–220°C barrel temperature; include a vacuum vent downstream of the reactive zone to remove unreacted MAH and peroxide decomposition products; pelletise and QC test (FTIR grafting degree, MFI, crystallisation point of residual MAH <0.5%). For the MAH grade, use standard grade (purity ≥99.0%, crystallisation point ≥52.5°C, moisture-free, fine flakes). Low colour is not critical for opaque GF-PP or natural fibre applications. The most important parameters are: consistent purity (affects GD consistency), uniform particle size (affects pre-blend dispersion), and dry storage (prevents maleic acid formation that reduces grafting efficiency). Sinolook Chemical supplies standard MAH in 25 kg PE-lined bags, 500 kg big bags, and on request as fine powder for improved pre-blend distribution - contact sales@sinolookchem.com or WhatsApp 0086 18150362095 to discuss grafting-grade specifications for your application.