Composition: MAH + orthophthalic anhydride (OPA/PA) + propylene glycol (PG)

Market share: ~50–60% of global UPR production; the largest and most commoditised type

• Lowest cost UPR type due to low PA price
• Good mechanical properties for most construction and marine applications
• Moderate water resistance (PA ester bonds are hydrolysable long-term)
• Not recommended for potable water contact or prolonged water immersion (>3 years)

Composition: MAH + isophthalic acid (IPA) + neopentyl glycol (NPG) or PG

Market share: ~25–30% of UPR; premium-grade type

• Superior water and chemical resistance vs orthophthalic (isophthalic ester bonds more hydrolysis-resistant)
• Preferred for marine gel-coat and boat hull structural laminates
• Better UV stability and gloss retention
• Higher cost due to IPA price premium over OPA
• Standard for chemical process equipment, potable water tanks

Composition: MAH + dicyclopentadiene (DCPD) + PA + PG (DCPD reacts with MAH via Diels-Alder + ene reaction)

Market share: ~15–20% of UPR

• DCPD is a low-cost C₅ fraction by-product from ethylene crackers
• DCPD-UPR has very low styrene content (15–25%) - reduced VOC emissions during moulding
• Lower viscosity facilitates spray-up and hand lay-up application
• Widely used in bathroom sanitary ware (bathtubs, shower trays) and automotive SMC

Styrene serves as both viscosity reducer and crosslinker. Each styrene molecule bridges two backbone C=C bonds (one in each polyester chain), creating the 3D network. Typical styrene:polyester ratio of ~1:2 by weight gives a network with approximately one crosslink per 2–3 MAH units in the backbone. Styrene content strongly affects gel time, shrinkage, and VOC emissions.

Methyl ethyl ketone peroxide (MEKP) is the standard room-temperature initiator for hand lay-up UPR. It decomposes (catalysed by the cobalt drier) to generate free radicals that initiate the styrene/polyester copolymerisation. Other initiators used for elevated-temperature cure: dibenzoyl peroxide (paste form), tert-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate. Initiator type and concentration control gel time and peak exotherm temperature.

Cobalt neodecanoate or cobalt octoate (isooctanoate), typically supplied as 6–12% Co solution in mineral spirit, catalyses the decomposition of MEKP to free radicals via a Fenton-type mechanism (Co²⁺/Co³⁺ redox cycle). Without cobalt, MEKP will not initiate polymerisation at room temperature. The cobalt level controls the induction period and gel time - higher cobalt = shorter gel time but lower exotherm peak. Sinolook Chemical also supplies cobalt drier solutions based on neodecanoate ligand for UPR applications.

Gel time	8–30 min (typical)
Peak exotherm	80–150°C (thin laminates)
Tack-free time	30–90 min after gel
Full cure	24 h at RT or post-cure at 60–80°C

⚠️ Impact of MAH purity on stoichiometry: If MAH contains 0.3% maleic acid, you are inadvertently adding extra –COOH groups (from maleic acid, MW 116) in the form of a diacid rather than anhydride. This shifts the OH:COOH ratio toward deficient glycol: effectively 1.04:1 instead of 1.05:1. At high synthesis temperatures, this leads to a slightly higher final AN and lower molecular weight than intended. For consistent batch-to-batch UPR properties, the MAH purity and maleic acid content from the COA must be used to correct the glycol charge when outside the normal range.

UPR Type	MAH Grade	Key Spec
General-purpose ortho	Standard	≥99.0%; APHA ≤30
Clear gel-coat / translucent	Premium ⭐	≥99.5%; APHA ≤10; Fe ≤1 ppm
Marine isophthalic	Premium ⭐	≥99.5%; APHA ≤10; Fe ≤1 ppm
Chemical pipe/tank	Standard	≥99.0%; APHA ≤20

• 25 kg PE-lined bags: Standard; easy to handle; suitable for small–medium batch reactors; stack on pallets under cover
• 500 kg / 1,000 kg big bags (FIBC): Cost-effective for ≥5 MT orders; requires FIBC discharger station above melt tank
• Molten ISO tank (heated): For large UPR plants processing >100 MT MAH/month; eliminates manual bag handling; molten MAH pumped directly to reactor; most economical and safest option at scale
• DG Class 4.1 (solid), UN 2215 for all packaging types

Q1: What is the role of maleic anhydride in unsaturated polyester resin?

Maleic anhydride serves as the essential source of reactive carbon-carbon double bonds (C=C) in the polyester backbone of unsaturated polyester resin (UPR). In UPR synthesis, MAH reacts with glycols (propylene glycol, ethylene glycol) by ring-opening and subsequent polycondensation to form a polyester chain containing maleate/fumarate units - segments with the structure –OOC–CH=CH–COO– in the backbone. These backbone double bonds are the crosslinking sites that react with styrene monomer during curing (initiated by MEKP peroxide and cobalt accelerator) to form a three-dimensional thermoset network. Without MAH, the polyester would have no unsaturation, no ability to crosslink with styrene, and would remain a thermoplastic with no structural utility for fibreglass composites. The MAH:PA (phthalic anhydride) ratio controls crosslink density: higher MAH fraction = more crosslinks = harder, more chemically resistant cured resin; lower MAH fraction = fewer crosslinks = softer, tougher, cheaper product. Global UPR production consumes approximately 40–50% of total MAH production worldwide.

Q2: What is the difference between orthophthalic and isophthalic UPR, and how does MAH content differ?

Orthophthalic (ortho) UPR uses ortho-phthalic anhydride (OPA, CAS 85-44-9) as the saturated diacid component, while isophthalic (iso) UPR uses isophthalic acid (IPA, CAS 121-91-5). MAH is used in both types as the unsaturated (crosslinkable) component - its content and role are essentially the same. The key difference is in the non-MAH acid component: orthophthalic resin is the commodity standard (lower cost, adequate performance); isophthalic resin commands a premium because isophthalic ester bonds are more resistant to hydrolysis than orthophthalic ester bonds under prolonged water immersion. For boat hulls, marine gel-coat, and chemical plant equipment exposed to continuous water contact, isophthalic grade is specified. Both types typically contain MAH in a molar ratio of approximately 1:1 to 1:2 with the saturated acid. Isophthalic UPR formulations often also use neopentyl glycol (NPG) instead of propylene glycol for further improved hydrolytic stability. The MAH content and quality requirements (especially colour and iron content) are similar for both grades - but gel-coat-grade isophthalic UPR has the strictest requirements (APHA ≤10 molten, Fe ≤1 ppm) due to colour sensitivity.

Q3: How does MAH iron content affect UPR quality?

Iron contamination in MAH (from reactor corrosion or equipment contact) has two distinct negative effects on UPR quality: (1) Colour: Iron in MAH is carried through into the polyester synthesis and produces yellow-to-brown discolouration in the final resin. Even trace levels of iron (above ~5 ppm) are visible in clear or white gel-coat formulations, causing rejection by quality-conscious customers. This is the most common colour-related quality failure in UPR; (2) Premature gelation: Iron is a radical initiator and can catalyse premature polymerisation of styrene during the MAH dissolution or reactor charging stages. In worst cases, iron-contaminated MAH added to a styrene-diluted UPR can cause gelation in the storage tank - a costly batch loss. Iron also consumes the polymerisation inhibitor (hydroquinone, methyl hydroquinone) in the styrene phase, reducing the shelf life of styrene-dissolved UPR from months to weeks. For premium gel-coat and marine UPR applications, MAH iron content should be ≤1 ppm; for standard structural laminates, ≤5 ppm is acceptable. If a batch of UPR develops unexpected colour or gel time problems, iron in the MAH should be the first suspect - request ICP analysis of the retained MAH batch sample from your supplier.

Q4: Why is a nitrogen atmosphere important during UPR synthesis?

Nitrogen (N₂) atmosphere serves four critical functions during UPR synthesis: (1) Water removal: The polycondensation reaction is equilibrium-limited; water must be continuously removed to drive the reaction toward higher molecular weight polyester. A sweep of nitrogen through the reactor head space strips water vapour from the melt surface and carries it to the condensate trap. Without N₂ sweep, the reaction equilibrium limits the final degree of polymerisation and leaves a high Acid Number polyester; (2) Preventing MAH sublimation/vapour loss: At synthesis temperatures, MAH has non-negligible vapour pressure; a nitrogen blanket reduces partial pressure of MAH vapour and minimises product loss to the condenser system; (3) Preventing colour development: Oxidation of the maleate/fumarate units at high temperature in air generates yellow/brown chromophores; nitrogen atmosphere prevents air oxidation entirely; (4) Safety: At synthesis temperatures (160–220°C), the liquid polyester/glycol mixture is well above the flash points of propylene glycol (99°C) and approaching that of MAH (102°C); maintaining an inert nitrogen atmosphere eliminates the risk of ignition of vapour above the reactor melt surface. N₂ is typically applied from the beginning of heating and maintained throughout synthesis, cooling, and styrene dilution.

Q5: What Acid Number should I target for my UPR synthesis?

The target Acid Number (AN) for UPR synthesis depends on the intended application and processing requirements. AN is expressed as mg KOH per gram of polyester and reflects the residual unreacted carboxylic acid groups. Lower AN = higher degree of polymerisation = higher molecular weight = higher viscosity melt and longer gel time in the cured system. Typical targets by application: Chemical-resistant pipe/tank UPR: AN 20–28 mg KOH/g - high MW polyester, maximum crosslink potential, best chemical resistance; Standard structural laminate (marine, construction): AN 28–35 mg KOH/g - balanced properties; General-purpose moulding/casting: AN 35–45 mg KOH/g - lower MW, lower viscosity, easier processing; DCPD-modified or flexible UPR: AN up to 50 mg KOH/g - lower MW intentional for specific processing. AN is typically monitored during synthesis by withdrawing a small sample every 15–30 minutes during the polycondensation stage, cooling to solid, dissolving in acetone, and titrating with standardised KOH solution. Most UPR producers have a titration endpoint of within ±3 mg KOH/g of target. After reaching target AN, viscosity at 75°C (ICI plate-cone) is checked as a secondary parameter - if AN is in range but viscosity is low, the polyester may have had excessive glycol that needs to be removed under stronger vacuum.

Q6: Can Sinolook Chemical supply MAH for large-scale UPR production in both bags and molten form?

Yes - Sinolook Chemical supplies maleic anhydride (CAS 108-31-6) for UPR production in three packaging formats: 25 kg PE-lined kraft bags (standard; minimum order 1 MT; suitable for batch reactors up to ~5 MT MAH per production run); 500 kg or 1,000 kg FIBC big bags (for orders of 5–20 MT; requires FIBC discharger and melt vessel at the receiving facility; most economical bag packaging for medium-scale UPR producers); Heated ISO tank (molten bulk, ~18–20 MT) (for large UPR plants with a heated MAH storage tank and pumping system; molten MAH at 65–75°C is pumped directly from the ISO tank into the reactor; eliminates bag handling, dust risk, and manual melting; lowest per-MT price). All packaging complies with IMDG Class 4.1, UN 2215, PG III. We supply standard grade (purity ≥99.0%, APHA ≤30 molten) for most structural UPR applications, and premium grade (≥99.5%, APHA ≤10, Fe ≤1 ppm) for gel-coat and marine-grade UPR. Full batch COA, GHS SDS, REACH OR letter, TSCA certification, and DG documentation are provided with every shipment. Contact: sales@sinolookchem.com or WhatsApp 0086 18150362095.