DOTP Grade Selection: A Technical Procurement Guide

DOTP (dioctyl terephthalate) is commonly supplied in three grades: superior, first-grade and qualified. When purchasing, do not stop at the grade name. Check the actual specification values.

The main differences between the three grades are colour and volume resistivity; the other six specification rows are identical. In practice, the right grade follows the appearance of the finished part, the electrical requirements and the production cost.

This guide covers grade selection from four angles: specification, application, purchase-order requirements and arrival inspection. It helps buyers match a grade to actual production needs.

Step one: understand the spec differences between the three DOTP grades

All three grades share the same chemical structure and CAS number, 6422-86-2. Comparing the specification rows shows that the grade split turns on colour and volume resistivity; the remaining rows are essentially identical.

1. Colour: how the finished part looks

Colour is reported on the APHA scale. The lower the number, the paler the liquid.

• Superior: ≤150 APHA
• First-grade: ≤230 APHA
• Qualified: ≤450 APHA

For transparent, pale or appearance-critical PVC parts, start with superior or first-grade. For black and dark-coloured goods, the plasticizer's own colour normally has no visible effect on the final appearance, so the qualified grade can be selected as the application allows.

2. Volume resistivity: the electrical requirement

Superior and first-grade both specify volume resistivity of ≥2×10¹² Ω·cm; the qualified grade specifies ≥2×10¹¹ Ω·cm.

Where the product carries a clear insulation requirement, check this row first and choose a grade that meets the resistivity requirement. Liquid colour alone never shows whether electrical performance is met.

Colour and resistivity are separate test items and cannot stand in for each other. Confirm each against the product's requirements.

Superior

≤150 APHA ceiling, arrives close to colourless

First-grade

≤230 APHA ceiling, arrives transparent pale yellow

Qualified

≤450 APHA ceiling, arrives yellow

Step two: choose the grade by application

Different PVC products put different demands on DOTP. Confirm the colour and electrical requirements of the finished product first, then fix the grade.

Cable sheaths and black goods

For black cable jackets and sheaths, dark moulded parts and similar products without an extra resistivity requirement, the qualified grade is normally the right choice.

The final colour of these parts comes mainly from the pigment; the plasticizer's own colour has little effect on appearance. There is no reason to pay for a lower APHA ceiling for its own sake.

But where the cable compound or another insulation product carries a defined volume resistivity requirement, the electrical requirement governs. The part being black does not remove that parameter.

Transparent parts and appearance-critical products

String-light skins, transparent lighting plastics and other pale PVC parts are more sensitive to the plasticizer's colour.

Because the plasticizer's own colour can show through the part, first-grade or superior is the safer choice; where transparency and colour carry the highest requirements, choose superior and its lower APHA ceiling.

PVC film

Grade selection for PVC film follows the film's colour, thickness, processing route and end use.

For dark films or products with no requirement on raw-material colour, the qualified grade normally meets the need; where transparency or a pale appearance matters, consider first-grade or superior.

PVC gloves

PVC gloves can run on the qualified grade, or on first-grade or superior when the customer requires it.

Ester content, acid value, flash point, moisture and density are the same across the three grades; the differences are colour and volume resistivity. If the customer accepts the qualified band's colour range and the formulation carries no higher resistivity requirement, the qualified grade is a valid purchase.

Where the customer sets stricter requirements on glove appearance and colour, choose first-grade or superior. The customer's acceptance criteria and actual production test results decide.

Where the product matches none of the applications above, work down this order:

1. First confirm whether a volume resistivity requirement applies.
2. Then confirm the colour and transparency requirements of the finished product.
3. Finally settle the grade against purchase cost.

Step three: what the purchase order should state

When buying DOTP, do not write only “superior”, “first-grade” or “qualified” on the order. Put the actual specification values into the purchase contract or order, so both sides read the same grade.

Make sure the order covers the following:

1. State the grade and the colour ceiling

For example:

Qualified DOTP, colour ≤450 APHA.

Orders for superior or first-grade should likewise state the matching APHA ceiling.

2. State the volume resistivity requirement

Where the product has an electrical requirement, state it on the order:

Volume resistivity ≥2×10¹² Ω·cm.

Note in particular that the qualified grade is specified at ≥2×10¹¹ Ω·cm and cannot be accepted against the superior or first-grade value.

3. State the other base specification rows

List the following parameters in the purchase documents as well:

• Ester content: ≥98%
• Acid value: ≤0.035 mg KOH/g
• Flash point: ≥210°C
• Moisture: ≤0.1%
• Density: 0.980–0.985 g/cm³ (20°C)
• CAS: 6422-86-2

4. Set the sample and acceptance criteria

Before a volume order, ask the supplier for samples for trial blends and production tests. Once a sample is approved, the approved sample and its specification values become the acceptance basis.

This cuts the risk of colour deviations or off-spec values after bulk arrival.

Step four: check the batch test report on arrival

Arrival inspection cannot rest on appearance alone; ask for the COA (certificate of analysis) for every batch.

Check the following:

Colour result

Compare the APHA value measured on the COA with the ceiling agreed in the purchase contract. For a qualified-grade order, the colour must meet ≤450 APHA.

Volume resistivity result

Where the order carries a resistivity requirement, check the measured value in the report against the contractual floor.

Base specification rows

Check ester content, acid value, flash point, moisture and density against the purchase specification as well.

Batch and sample records

Keep each batch's COA, the approved-sample records and the purchase information on file for quality traceability. If colour or performance differs between batches, the data is there to compare.

Product colour is a first visual check only; final acceptance follows the contractual specification values and the test results.

Frequently asked questions

There is no need to chase the highest grade on principle. Match the grade to the application, the customer's standard, the key specification values and the actual cost, and control quality through sample testing and batch COAs.