A Technical Data Sheet (TDS) packs a resin grade into one page of numbers. Read it well and you can tell, before ordering a sample, whether a grade will flow into your reinforcement and give you enough working time. Read it badly and you end up comparing figures that were never measured the same way. This guide explains the properties you will find on unsaturated polyester resin data sheets, what each one tells you, and which ones appear on a VRP data sheet today.
If you are unsure what a TDS is for compared with an MSDS or a CoA, start with our guide TDS vs MSDS vs CoA.
Two kinds of property: liquid and cured
Data sheet properties fall into two groups.
- Liquid properties describe the resin as it arrives in the drum. They tell you how it will handle and cure in your shop.
- Cured properties describe a test piece after the resin has hardened. They give a guide to how the finished part may perform.
Data sheets vary in how much they cover. Some list only liquid properties. Others add cure-behaviour tests and cured properties. Either way, the figures are typical values for the grade, not measurements of your batch. Your batch is reported on the CoA that comes with every dispatch.
Liquid properties on a VRP data sheet
Appearance. Colour and clarity of the liquid resin. It matters most for clear casting and light-coloured parts, where a yellow tint will show.
Viscosity. How thick the resin is, reported in centipoise (cP) or millipascal-seconds (mPa·s), which are the same unit. Low viscosity flows and wets reinforcement quickly. High viscosity resists running on vertical surfaces. Viscosity changes sharply with temperature, so a figure is only meaningful with its test temperature and method beside it. For more, see our article on high- and low-viscosity resins.
Acid value. Reported in mg KOH/g. It is a measure of the acid groups left in the polymer, and a manufacturer uses it to confirm that each batch has been made consistently. For a buyer, its main use is batch-to-batch comparison on the CoA. It is not a performance figure on its own.
Specific gravity. Density relative to water. You need it to convert between litres and kilograms, and to estimate how much resin a part will use.
Gel time. The time from adding catalyst until the resin turns from liquid to a soft gel. This is your working window. A gel time is only meaningful with its test conditions: temperature, catalyst type and dose, and accelerator dose if you add your own. The same resin will gel faster on a hot afternoon than on a cool morning.
Volatile, or Non-volatile (solid) content. Volatile: The share of the resin that is reactive monomer (usually styrene); Non-Volatile: The share of the resin that is polymer rather than reactive monomer. It affects viscosity, shrinkage and styrene emission.
Shelf life and storage. How long the unopened resin stays usable, and at what temperature. Pre-accelerated grades usually carry a shorter shelf life than their non-accelerated equivalents.
Properties some data sheets add
You may see these on other suppliers’ data sheets. It helps to know what they mean.
Thixotropic index. Found mainly on gelcoats and some laminating resins. A thixotropic resin is thick when still and thins when stirred or sprayed, then thickens again on the mould. A higher index means stronger sag resistance.
Peak exotherm and gel-to-peak time. Curing gives off heat. Peak exotherm is the highest temperature reached in a standard test mass; gel-to-peak is how long it takes to get there. High exotherm in thick sections can cause cracking, warping or discolouration.
Cured properties. These are usually measured on a cast specimen: resin alone, with no glass fibre.
- Heat deflection temperature (HDT): the temperature at which a loaded test bar bends by a set amount. A guide to heat resistance.
- Tensile strength, modulus and elongation at break: how strong and stiff the cured resin is, and how far it stretches before breaking. Elongation is a useful indicator of toughness.
- Flexural strength and modulus: resistance to bending.
- Barcol hardness: surface hardness after cure. Often used on the shop floor to check whether a part is cured enough to demould.
- Water absorption: how much water a cured specimen takes up in a set time. Relevant for marine, tank and water-contact parts.
A laminate with glass reinforcement will perform very differently from a cast specimen. Use cured figures to compare grades with each other, not to design a part.
Why a VRP data sheet focuses on liquid properties
Two suppliers can publish different gel times for resins that behave the same, simply because they tested at different temperatures or catalyst doses. Good data sheets name the method, usually an ISO or ASTM standard, next to each figure. Examples include ISO 2555 for Brookfield viscosity, ISO 2114 for acid value and ISO 2535 for gel time at ambient temperature. Cured properties have their own methods, such as ISO 75 for heat deflection temperature.
When you compare two data sheets, line up the methods and conditions first. If they differ, ask the supplier to test both resins the same way, or run your own side-by-side trial.
Quick Reference
| Property | Unit | What it tells you | What to check | On a VRP TDS today |
|---|---|---|---|---|
| Appearance | visual | Colour and clarity | Matters for clear and light parts | Yes |
| Viscosity | cP or mPa·s | Flow and wet-out | Test temperature and method | Yes |
| Acid value | mg KOH/g | Batch consistency | Compare CoA to specification | Yes |
| Specific gravity | none | Litres-to-kg conversion | Temperature of measurement | Yes |
| Gel time | minutes | Working window | Catalyst dose and temperature | Yes |
| Volatile content | % | Monomer share | Relevant to emissions and shrink | Yes |
| Shelf life | months | Usable storage period | Storage temperature | Yes |
| Thixotropic index | ratio | Sag resistance | Mainly for gelcoats | No, ask us |
| Peak exotherm | °C | Heat build-up in cure | Section thickness in your part | No, ask us |
| HDT | °C | Heat resistance | Cast specimen, not laminate | No, ask us |
| Tensile / flexural strength | MPa | Strength of cured resin | Cast specimen, not laminate | No, ask us |
| Barcol hardness | Barcol units | Degree of cure | Instrument model used | No, ask us |
Read the test method, not just the number
Two suppliers can publish different gel times for resins that behave the same, simply because they tested at different temperatures or catalyst doses. Good data sheets name the method, usually an ISO or ASTM standard, next to each figure. Examples include ISO 2555 for Brookfield viscosity, ISO 2114 for acid value and ISO 2535 for gel time at ambient temperature. Cured properties have their own methods, such as ISO 75 for heat deflection temperature.
When you compare two data sheets, line up the methods and conditions first. If they differ, ask the supplier to test both resins the same way, or run your own side-by-side trial.
Which figures matter most for you
- Hand lay-up and spray-up: viscosity and gel time, plus how thixotropic the resin is. If the TDS gives no thixotropic index, ask.
- Casting and thick sections: gel time, plus peak exotherm and shrinkage. Where these are not on the data sheet, ask your supplier for guidance on thick sections.
- Infusion and resin transfer moulding: viscosity at your shop temperature, and gel time long enough to fill the part.
- Parts facing water, heat or chemicals: the backbone chemistry first, then cured properties if your specification calls for them. See Choosing the Right Polyester Resin Backbone.
At Vikram Resins
Every VRP grade has a TDS, and the grade code on it tells you the backbone, accelerator status and viscosity class before you read a single number. See How to Read a VRP Grade Code. If you already have a data sheet from your current resin, send it to us: it is the fastest way for us to match your requirement to a grade. Request any TDS from our products page, or contact our technical team.
FAQs
What is gel time in polyester resin?
The time from adding catalyst until the liquid resin turns into a soft gel. It is your working window, and it depends on temperature and on the catalyst and accelerator doses used.
What does acid value mean on a resin data sheet?
A measure of the acid groups left in the polymer, in mg KOH/g. It is mainly a batch-consistency check, best compared on the CoA against the product specification.
Why is viscosity always given with a temperature?
Because resin thins as it warms and thickens as it cools. A viscosity figure without its test temperature cannot be compared with another.
Are cured properties on a TDS the same as my laminate's properties?
No. They are usually measured on cast resin without reinforcement. A glass-fibre laminate will have very different strength and stiffness.
Why doesn't a VRP data sheet list cured properties?
Our data sheets currently focus on the liquid properties we control on every batch. If your application depends on cured properties or exotherm, contact our technical team before you request a sample.