Buying a ready-made resin + hardener kit or buying MTHPA and BPA epoxy resin separately: manufacturing differences across end-use markets

 

Zhejiang Heyi Chemical Co., Ltd · MTHPA (CAS 11070-44-3) technical series
Audience APG moulders · engineers · buyers	Topic Procurement & formulation routes	Author Heyi Technical Service Dept.	Updated 23 Oct 2026

 

Key takeaways (summary for engineers and buyers)

Two routes: a pre-formulated APG epoxy system is a supplier's ready-made resin + hardener kit (often pre-filled and/or pre-accelerated, mixed at a fixed ratio); the single-component route means buying MTHPA, bisphenol-A (BPA) liquid epoxy resin, accelerator and silica separately and formulating in-house.

Formulation control: with MTHPA (anhydride equivalent weight ≈ 166 g/eq) and BPA resin of EEW ≈ 185–195 g/eq, a typical level is ≈ 80–90 phr MTHPA per 100 parts resin (anhydride/epoxy ratio ≈ 0.9–1.0); silica filler is typically 60–70 % by weight of the total mix.

Trade-off: single components give formulation freedom, component-level cost and dual-sourcing options, but transfer QC, process-window management and performance liability to the moulder. Pre-formulated systems shorten start-up and concentrate responsibility in one supplier.

End-use fit: CT/VT instrument transformers, bushings/insulators, GIS spacers and embedded poles are mainly made by APG; cast-resin dry-type transformer coils are mostly made by vacuum casting, not APG.

Rule of thumb: choose a pre-formulated system when speed, low volume or limited lab capability dominate; choose single components when volume is stable and you can control incoming QC, metering and gel-time testing.

01 Key definitions

The terms below are used throughout this article. Values marked "typical" are common industry ranges, not specifications; always use the supplier's technical data sheet (TDS) and certificate of analysis (COA).

Term	Definition
APG	Automatic Pressure Gelation: a degassed epoxy mix is injected at low pressure (typically 0.1–0.5 MPa) from the bottom into a heated steel mould (typically 130–160 °C); the part gels from the mould wall inward while pressure is held to compensate shrinkage.
Pre-formulated system	A supplier-designed resin component (Part A) and hardener component (Part B), often pre-filled with silica and/or pre-accelerated, used at a fixed mix ratio stated on the TDS.
Single-component route	Buying base raw materials separately (e.g. MTHPA, BPA liquid epoxy resin, accelerator, silica) and weighing, mixing and degassing them in-house to the moulder's own formulation.
MTHPA	Methyl tetrahydrophthalic anhydride, CAS 11070-44-3: a low-viscosity liquid anhydride hardener widely used in electrical casting and APG. Theoretical anhydride equivalent weight ≈ 166 g/eq.
EEW	Epoxy equivalent weight (g/eq): mass of resin containing one epoxy group. Standard BPA liquid resin is typically ≈ 185–195 g/eq (epoxy value ≈ 5.1–5.4 eq/kg).
phr	Parts per hundred parts of resin, by weight.
A/E ratio	Anhydride-to-epoxy equivalent ratio; anhydride systems are often formulated slightly below 1.0 (≈ 0.85–1.0, typical).
Pot life	Time the mixed, warm material (typically 40–60 °C) stays within the viscosity range the APG machine can inject.
Gel time	Time to gelation at a given temperature; in APG it sets demould time and depends strongly on accelerator level.
Free acid	Dicarboxylic acid formed when anhydride reacts with moisture; a key MTHPA quality indicator because it can shift reactivity and gel time.

02 What each route is

2.1 Route A: buying a pre-formulated APG epoxy system

A system supplier designs the chemistry and delivers two components. In many APG systems both parts are pre-filled with silica so that they can be mixed at a convenient fixed ratio (for example 1:1 by weight), and the hardener part is often pre-accelerated so that the moulder does not dose accelerator. Some systems are delivered unfilled, with the moulder adding silica at a stated ratio. The moulder's job is to preheat, homogenise, mix, degas and inject according to the TDS.

2.2 Route B: buying single components and mixing in-house

The moulder buys MTHPA, BPA liquid epoxy resin, accelerator and silica filler (plus optional flexibilisers, pigment paste or additives) from one or more suppliers, verifies each lot, and formulates its own mix. This requires heated storage, accurate metering (especially for the accelerator), premixing of filler, vacuum mixing/degassing and in-house gel-time and viscosity testing. In return the moulder controls the recipe and the bill of materials.

Figure 1 | The two procurement and production routes (schematic)

Figure 2 | An APG moulding shop: vacuum mixing vessel, clamping machines and a demoulded epoxy part  

03 Formulation control: stoichiometry, accelerators and filler

With single components, stoichiometry is the moulder's responsibility. The MTHPA level follows from the equivalent weights:

Stoichiometry formula

phr (MTHPA) = AEW ÷ EEW × 100 × r

AEW ≈ 166 g/eq for MTHPA (theoretical; the practical value is derived from the lot's anhydride content); EEW ≈ 185–195 g/eq for standard BPA liquid resin; r = A/E ratio.

Example: EEW 190, r = 1.0 → 166 ÷ 190 × 100 ≈ 87 phr; r = 0.9 → ≈ 79 phr. This is why ≈ 80–90 phr is the commonly quoted range.

Figure 3 | Stoichiometry infographic: calculated MTHPA phr for EEW 185–195 and an illustrative filled APG mix

 

■ Why it matters: because each resin lot has its own EEW and each MTHPA lot its own anhydride content, an in-house formulator can correct the ratio lot by lot. A pre-formulated system hides this adjustment inside the supplier's process.

■ Accelerators: tertiary amines (e.g. benzyldimethylamine, BDMA) and imidazoles (e.g. 1-methylimidazole, 2-ethyl-4-methylimidazole) are common. Dosage is small, typically a fraction of a phr up to about 1–2 phr, and is set by gel-time trials rather than stoichiometry; small weighing errors change gel time noticeably.

■ Filler: silica flour typically makes up 60–70 % by weight of the total mix (for 100 resin + 85 MTHPA that is roughly 280–430 parts of filler). It lowers cost, shrinkage, exotherm and thermal expansion but raises viscosity, so a low-viscosity hardener such as MTHPA helps keep the mix injectable. Silane-treated grades are common; filler must be dried and preheated.

■ Other levers: flexibilisers or toughened resins for crack resistance around inserts; alumina instead of silica where SF₆ decomposition products are a concern (GIS); pigment pastes. Each additional component is another item to qualify and control.

04 Cost structure

Cost comparisons are often reduced to price per kilogram, which can mislead. The two routes distribute cost differently:

Cost element	Route A: pre-formulated system	Route B: single components
Material price	Includes the supplier's formulation know-how, blending, QC, packaging and support	Component prices; filler bought separately, often locally
Freight on filler	With pre-filled parts, freight is paid on the filler share as well	Filler can be sourced near the plant
Equipment	Mixing/degassing equipment for A+B	Additional heated storage, metering units, filler drying/premixing, lab instruments
People	Process operators	Plus formulation/lab staff and incoming-QC testing time
Inventory	Two SKUs per system	More SKUs; separate shelf lives and storage conditions
Risk cost	Supplier batch issues are handled by the supplier	Weighing errors, wrong lots or moisture pick-up become in-house scrap risk
Change cost	Changing system supplier usually triggers requalification	Changing one component still needs equivalence testing

In practice, the material saving of Route B grows with volume, while its fixed costs (lab, QC, metering) are largely independent of volume. That is why the break-even point is plant-specific and should be calculated with your own volumes, scrap rate and labour costs rather than taken from a generic figure.

05 Supply chain and dual-sourcing flexibility

■ Route A ties the moulder to one system supplier. Because end customers often type-test and approve a specific material, a change of system usually means new tests and approvals. Continuity depends on that supplier's capacity and logistics.

■ Route B allows each component to be qualified from two or more sources. MTHPA and BPA liquid resin are produced by many manufacturers, which reduces single-point risk. However, products sold under the same name can differ in viscosity, isomer composition, free acid, colour or EEW, so a second source still needs equivalence testing (gel time, viscosity, Tg and electrical properties).

■ Change notification: whichever route you choose, agree with suppliers that process or raw-material changes are notified in advance; for type-tested insulation products, also inform your end customer as their rules require.

06 QC burden: what has to be checked

Route A mainly requires checking the supplier's COA for each part (for example viscosity, reactivity or gel time, and filler content for pre-filled parts), storage conditions and shelf life, plus re-homogenising pre-filled parts in which filler may settle. Route B requires incoming inspection for every component and a check of the mixed system:

Material	Typical incoming checks (COA + in-house)	Why it matters
MTHPA	Appearance; colour (Pt-Co/Hazen; typical spec ≤ 100–200); viscosity at 25 °C (typical ≈ 30–80 mPa·s); anhydride content (typical ≥ 40.5–41.5 %); acid value (typical ≈ 660–685 mg KOH/g); free acid (typical ≤ 0.5–1.0 %); volatiles	Anhydride content sets the real equivalent weight; free acid and moisture shift gel time; viscosity affects filled-mix flow
BPA liquid epoxy resin	EEW / epoxy value (typical EEW ≈ 185–195 g/eq); viscosity at 25 °C; colour; volatiles; hydrolysable chlorine where specified	EEW sets the MTHPA phr; viscosity affects mixing and filler loading
Accelerator	Identity/assay; water content; colour	Tiny doses: purity or water changes reactivity
Silica / alumina filler	Moisture; particle-size distribution; surface treatment	Moisture causes voids and reacts with anhydride; PSD drives viscosity
Mixed system	Gel time at a fixed temperature (e.g. 150 °C); viscosity at mixing temperature; periodic Tg on cured samples	Final confirmation that the lot combination stays in the process window

Figure 4 | In-house metering, mixing and degassing line with QC check points (schematic)

 

07 Process windows: pot life, gel and post-cure

Stage	Typical condition	Route A	Route B
Mixing & degassing	Vacuum, typically 40–60 °C	Window defined by the supplier's TDS; pre-accelerated parts start reacting once mixed	Moulder chooses accelerator type/level to balance pot life against gel speed
Pot life	Hours at mixing temperature, depending on accelerator (verify by viscosity–time curve)	Fixed by the system; little room to adjust	Adjustable, but must be re-validated after each change
Injection & gel	0.1–0.5 MPa; mould typically 130–160 °C; demould often 20–60 min	Supplier recommends mould temperature and gel time	Moulder tunes gel time with accelerator and mould temperature
Post-cure	Several hours in an oven (typical schedules around 130–140 °C; follow TDS)	Schedule given by supplier	Schedule developed and validated in-house (Tg, mechanical, electrical tests)

 

Risk note: batch variation = gel-time variation = mould scrap risk

In APG the shrinkage compensation depends on a stable gel front. If the hardener's acid value, free acid, colour or viscosity drifts from lot to lot, gel time drifts too, which can cause voids, cracks or a scrapped mould load. Under Route B this risk is managed by the moulder; consistent MTHPA lots make it much easier to manage.

08 Technical support and liability

 

■ Route A: the system supplier typically supports qualification, processing trials and troubleshooting, and stands behind the system's properties when it is processed as specified. Root-cause discussions are simpler because one party owns the formulation.

■ Route B: the moulder owns the formulation and therefore the part's material performance. Component suppliers are normally responsible for conformity of their product to its agreed specification and COA, and can support with application data, but not for a recipe they did not design.

■ Practical tip: for Route B, agree on specifications, test methods and change notification in a quality agreement with each component supplier. (This article is technical guidance, not legal advice.)

09 Side-by-side comparison

 

Dimension	Route A: pre-formulated system	Route B: single components
What you buy	Part A + Part B kit (often pre-filled / pre-accelerated)	MTHPA, BPA resin, accelerator, filler separately
Formulation control	Low: fixed by supplier	High: own stoichiometry, accelerator, filler
Mix ratio handling	Fixed ratio, simple weighing	Multiple components; accelerator needs precise dosing
Cost basis	Higher price per kg, lower fixed cost	Lower material cost, higher fixed cost
Supply chain	Single system source; change needs requalification	Component-level dual sourcing possible
QC burden	COA review, storage, re-homogenising	Full incoming QC plus mixed-system gel-time tests

Process window	Defined by supplier	Tunable but must be validated in-house
Start-up time	Short	Longer (formulation development, validation)
Technical support	System-level support from supplier	Component data from suppliers; system know-how in-house
Liability	Concentrated at system supplier (within spec use)	Formulation performance owned by moulder
Best fit	New lines, low or variable volume, limited lab	Stable high volume, strong lab and metering capability

 

Figure 5 | Qualitative comparison of the two routes (illustrative relative judgement, not measured data)

 

10 End-use scenarios

The right route also depends on what is being made. The table and map below summarise typical practice; individual plants and customer specifications vary.

Figure 6 | End-use application map: product, typical process, material needs and common route (indicative)

 

Application	Typical process	Material considerations	Route considerations
CT / VT instrument transformers	APG	Many models and high volume; low partial discharge, crack resistance around windings	Large plants with stable volume often formulate in-house; smaller or newer lines often start with a system
Epoxy bushings & insulators	APG	Indoor parts commonly BPA + anhydride + silica; outdoor parts often need cycloaliphatic or other weather-resistant systems	Route depends on indoor/outdoor and on the customer's approved material
GIS spacers	APG	SF₆ service: alumina filler is common because silica can be attacked by SF₆ decomposition products; high mechanical and dielectric demands	Strict qualification; changes are costly, so many stay with an approved system; in-house mixing is feasible with a qualified recipe
Embedded poles & switchgear parts	APG	Metal inserts and vacuum interrupters; thermal-cycle crack resistance; toughened formulations	Either route; in-house mixers must control toughener and accelerator dosing tightly

Cast-resin dry-type transformers

Mostly vacuum casting (not APG)

Large coils, long cure, exotherm and thermal-cycling crack resistance

Batch vacuum mixing of single components is common; systems are also used

 

Figure 7 | Typical cast-epoxy insulation parts: GIS-type spacer, post insulator, embedded pole and current-transformer body

11 Who should choose which route?

■ Choose a pre-formulated system (Route A) if you are starting a new APG line or product, your volume is low or fluctuates, you lack a lab that can test gel time, viscosity, EEW and acid value, or your end customer has approved a specific system.

■ Choose single components (Route B) if your volume is stable and large, you have metering and vacuum-mixing equipment, trained formulation/QC staff, and you want cost control and dual sourcing at component level.

■ Consider a hybrid: many plants keep a system for critical, customer-frozen parts and formulate in-house for high-volume standard parts, or use a two-component resin/hardener pair and add their own filler.

Figure 8 | Decision tree for choosing between a pre-formulated system and single components

12 Where a reliable MTHPA supply adds value

For moulders on Route B, the hardener is the component that most directly drives reactivity, so its lot-to-lot consistency matters. A dependable MTHPA supplier helps by providing a COA for every lot covering acid value, free acid, anhydride content, viscosity and colour; keeping these values in a narrow band so that the phr and accelerator level do not need constant adjustment; packing to limit moisture pick-up; and giving advance notice of changes. Zhejiang Heyi Chemical Co., Ltd manufactures MTHPA (CAS 11070-44-3) for electrical casting and APG users and can support both routes: MTHPA as a single component for in-house formulators, and two-component resin/hardener systems for customers who prefer a ready formulation.

 

13 Frequently asked questions (FAQ)

 

Q: What is the difference between a pre-formulated APG epoxy system and buying MTHPA and epoxy resin separately?

A: A pre-formulated system is a supplier-designed resin + hardener kit, often pre-filled and pre-accelerated, used at a fixed ratio. Buying single components means you buy MTHPA, BPA resin, accelerator and filler separately and set the formulation, QC and process window yourself.

Q: How much MTHPA should be used per 100 parts of bisphenol-A epoxy resin?

A: Calculate phr = AEW ÷ EEW × 100 × r. With AEW ≈ 166 g/eq and EEW ≈ 185–195 g/eq, the result is typically ≈ 80–90 phr for an A/E ratio of about 0.9–1.0. Recalculate with the actual lot EEW and anhydride content and confirm by gel-time and Tg tests.

Q: Is buying single components always cheaper?

A: Not always. Material cost per kg is usually lower, but lab staff, QC testing, metering equipment, extra inventory and scrap risk are added. The balance depends on your volume and capabilities and should be calculated for your plant.

Q: Which incoming QC tests are needed for MTHPA?

A: Typically appearance, colour, viscosity at 25 °C, anhydride content, acid value, free acid and volatiles, checked against the supplier's COA, plus a gel-time test of the mixed system at a fixed temperature.

Q: Why does free acid in MTHPA matter for APG?

A: Free acid forms when the anhydride absorbs moisture. It changes reactivity and therefore gel time, and indicates how well the material has been protected from moisture. In APG, gel-time drift can lead to voids, cracks or a scrapped mould load.

Q: Can I switch from a pre-formulated system to in-house mixing without requalifying my product?

A: Usually you should expect to requalify. A new formulation can change Tg, mechanical strength, partial discharge and crack resistance, and type-tested products are typically approved for a specific material. Check with your end customer and the applicable standards.

Q: Are cast-resin dry-type transformers made by APG?

A: Mostly not. Their coils are large and are mostly made by vacuum casting with long cure schedules. APG is mainly used for CT/VT, bushings and insulators, GIS spacers and embedded poles.

Q: Which accelerators are used with MTHPA?

A: Tertiary amines such as BDMA and imidazoles such as 1-methylimidazole or 2-ethyl-4-methylimidazole are common. They are dosed in small amounts set by gel-time trials, so accurate metering is essential.

Q: Why are GIS spacers often filled with alumina instead of silica?

A: In SF₆ equipment, arcing can create decomposition products (including fluorine compounds) that can attack silica. Alumina is more resistant, so it is commonly used for SF₆-exposed parts.

Q: How should MTHPA be stored?

A: Keep containers tightly closed in a dry place, minimise air exposure after opening (a dry nitrogen blanket is common for bulk tanks), and follow the supplier's shelf life. Moisture converts anhydride to acid and raises free acid.

About the author and company Prepared by: Technical Service Department, Zhejiang Heyi Chemical Co., Ltd (浙江合益化学股份有限公司) Company: manufacturer of methyl tetrahydrophthalic anhydride (MTHPA, CAS 11070-44-3), a liquid anhydride hardener for epoxy electrical casting and APG, and of related epoxy resin systems. Address: No. 9 Lingxiu Road, Modern Industrial Park, Xianju County, Taizhou City, Zhejiang Province, China Tel: 0576-87899666 Published / updated: 23 October 2026 Disclaimer: values in this article are typical industry ranges or calculated examples, not product specifications. Figures 5 and 6 are qualitative; Figures 2 and 7 are AI-generated illustrations. Validate every formulation and process with your own trials, the supplier's TDS/COA and your end customer's requirements.