kraft paper

Description

Electrical grade kraft insulation paper is produced from 100% high‑grade sulfate (kraft) wood pulp, refined and formed to achieve low ash content, low ionic conductivity, and high cleanliness for use in liquid‑immersed and dry electrical insulation systems. Typical products are supplied in rolls or slit tapes with thicknesses from about 0.05 mm to 0.50 mm, providing flexibility for conductor covering, coil layer insulation, and interlayer insulation in transformer windings.

The material exhibits high dielectric strength (commonly ≥7–8 kV/mm in air), good tensile strength, and elongation sufficient to withstand winding and handling stresses without tearing. Kraft insulation paper is compatible with transformer oils and varnishes, showing good oil absorption and aging behavior; its long‑term performance is often evaluated by degree of polymerization (DPv) according to IEC 60450 and by system temperature limits under IEC 60076‑2 / IEC 60076‑14 for liquid‑immersed transformers.

Typical data table

Product name: Kraft paper Thickness:0.08mm Test time: 2026.1
No	Items		Unit	Requirements	Test Result	Test Method
1	Thickness		μm	±6.0%	3.75	IEC 60554-3:2023
2	Density		g/cm³	0.90±0.05	0.92
3	Tensile strength	Lengthwise	N·m/g	≥85.0	87.55
3	Tensile strength	Crosswise	N·m/g	≥38	39.34
4	Elongation	Lengthwise	%	≥2.0	2.46
4	Elongation	Crosswise	%	≥6.0	8.22
5	Ash content		%	≤0.7	0.42
6	Water content		%	6.0～9.0	6.55
7	Conductivity of aqueous extract		mS/m	≤10.0	6.82
8	PH of aqueous extract		—	6.5～8.5	7.3
9	Tear strength	Crosswise	mN·m²/g	≥7.50	9.2
10	Air Permeability		μm/Pa · s	≤0.50	0.24
11	Electrical Strength		kV/mm	≥7.5	8.92
Test result： Pass

Product name: Kraft paper Thickness:0.1mm Test time:2026.1
No	Items		Unit	Requirements	Test Result	Test Method
1	Thickness		μm	±6.0%	2.0	IEC 60554-3:2023
2	Density		g/cm³	0.90±0.05	0.92
3	Tensile strength	Lengthwise	N·m/g	≥85.0	87.64
3	Tensile strength	Crosswise	N·m/g	≥38	39.45
4	Elongation	Lengthwise	%	≥2.0	2.45
4	Elongation	Crosswise	%	≥6.0	8.21
5	Ash content		%	≤0.7	0.46
6	Water content		%	6.0～9.0	6.58
7	Conductivity of aqueous extract		mS/m	≤10.0	6.78
8	PH of aqueous extract		—	6.5～8.5	7.35
9	Tear strength	Crosswise	mN·m²/g	≥7.50	9.16
10	Air Permeability		μm/Pa · s	≤0.50	0.26
11	Electrical Strength		kV/mm	≥7.5	8.92
Test result： Pass

Product name: Kraft paper Thickness:0.13mm Test time: 2026.1
No	Items		Unit	Requirements	Test Result	Test Method
1	Thickness		μm	±6.0%	2.31	IEC 60554-3:2023
2	Density		g/cm³	0.90±0.05	0.93
3	Tensile strength	Lengthwise	N·m/g	≥85.0	88.15
3	Tensile strength	Crosswise	N·m/g	≥38	39.28
4	Elongation	Lengthwise	%	≥2.0	2.49
4	Elongation	Crosswise	%	≥6.0	8.36
5	Ash content		%	≤0.7	0.41
6	Water content		%	6.0～9.0	6.45
7	Conductivity of aqueous extract		mS/m	≤10.0	6.86
8	PH of aqueous extract		—	6.5～8.5	7.34
9	Tear strength	Crosswise	mN·m²/g	≥7.50	9.25
10	Air Permeability		μm/Pa · s	≤0.50	0.21
11	Electrical Strength		kV/mm	≥7.5	8.96
Test result： Pass

Product name: Kraft paper Thickness:0.18mm Test time: 2026.1
No	Items		Unit	Requirements	Test Result	Test Method
1	Thickness		μm	±6.0%	1.67	IEC 60554-3:2023
2	Density		g/cm³	0.90±0.05	0.93
3	Tensile strength	Lengthwise	N·m/g	≥85.0	79.35
3	Tensile strength	Crosswise	N·m/g	≥38	40.22
4	Elongation	Lengthwise	%	≥2.0	2.46
4	Elongation	Crosswise	%	≥6.0	8.43
5	Ash content		%	≤0.7	0.44
6	Water content		%	6.0～9.0	6.42
7	Conductivity of aqueous extract		mS/m	≤10.0	6.96
8	PH of aqueous extract		—	6.5～8.5	7.50
9	Tear strength	Crosswise	mN·m²/g	≥7.50	10.23
10	Air Permeability		μm/Pa · s	≤0.50	0.24
11	Electrical Strength		kV/mm	≥7.5	9.12
Test result： Pass

Product name: Kraft paper Thickness:0.25mm Test time:2026.1
No	Items		Unit	Requirements	Test Result	Test Method
1	Thickness		μm	±6.0%	2.10	IEC 60554-3:2023
2	Density		g/cm³	0.90±0.05	0.94
3	Tensile strength	Lengthwise	N·m/g	≥85.0	80.25
3	Tensile strength	Crosswise	N·m/g	≥38	42.23
4	Elongation	Lengthwise	%	≥2.0	2.45
4	Elongation	Crosswise	%	≥6.0	8.56
5	Ash content		%	≤0.7	0.45
6	Water content		%	6.0～9.0	6.54
7	Conductivity of aqueous extract		mS/m	≤10.0	6.85
8	PH of aqueous extract		—	6.5～8.5	7.50
9	Tear strength	Crosswise	mN·m²/g	≥7.50	10.36
10	Air Permeability		μm/Pa · s	≤0.50	0.26
11	Electrical Strength		kV/mm	≥7.5	9.45
Test result： Pass

Product name: Kraft paper Thickness:0.3mm Test time:2026.1
No	Items		Unit	Requirements	Test Result	Test Method
1	Thickness		μm	±6.0%	2.05	IEC 60554-3:2023
2	Density		g/cm³	0.90±0.05	0.94
3	Tensile strength	Lengthwise	N·m/g	≥85.0	81.04
3	Tensile strength	Crosswise	N·m/g	≥38	40.52
4	Elongation	Lengthwise	%	≥2.0	2.53
4	Elongation	Crosswise	%	≥6.0	8.64
5	Ash content		%	≤0.7	0.46
6	Water content		%	6.0～9.0	6.5
7	Conductivity of aqueous extract		mS/m	≤10.0	7.28
8	PH of aqueous extract		—	6.5～8.5	7.40
9	Tear strength	Crosswise	mN·m²/g	≥7.50	10.58
10	Air Permeability		μm/Pa · s	≤0.50	0.26
11	Electrical Strength		kV/mm	≥7.5	9.15
Test result： Pass

Note:The product specifications provided are for reference purposes only. Please refer to the actual product for final specifications. We also offer various sizes and thicknesses to meet your specific needs. For further customization, please contact us.

Application scenarios

Kraft insulation paper is widely used as conductor wrapping and layer insulation for low‑, medium‑, and high‑voltage transformer windings (power, distribution, and instrument transformers), where it provides electrical insulation, oil absorption, and mechanical protection. It is also used as slot, phase, and interturn insulation in electric motors and generators, as well as in capacitors and reactors where flexible cellulose insulation is required.

In cable manufacturing, electrical cable insulating kraft paper is applied as lapped insulation around conductors, benefiting from its dielectric strength, flexibility, and moisture‑treated grades that improve resistance to humidity and environmental stress. It is suitable for use in oil‑filled power cables, communication cables, and control cables, often in combination with other papers, films, or barrier materials as part of a composite insulation system.

Testing standards and quality control

Key electrical and chemical properties of kraft insulation paper—such as dielectric strength, dissipation factor, and conductivity—are typically tested according to IEC and related standards specific to paper and transformer/cable insulation. The degree of polymerization (DPv), which indicates mechanical integrity and aging condition of kraft paper, is evaluated using viscometric methods standardized in IEC 60450, widely applied for both new and aged transformer insulation.

For transformer applications, kraft paper is part of a cellulose/oil insulation system whose thermal performance and temperature limits are governed by IEC 60076‑2 and IEC 60076‑14 for liquid‑immersed transformers, distinguishing conventional kraft‑based systems from high‑temperature insulation alternatives. Additional quality parameters such as ash content, aqueous extract conductivity, pH, and heat stability are often specified in supplier technical data sheets to ensure long‑term reliability and compatibility with insulating liquids.

FAQs (with standards/technical context)

Q1: What is electrical kraft insulation paper made from?
A1: Electrical kraft insulation paper is produced from high‑purity, unbleached sulfate (kraft) wood pulp, refined to achieve low ash content, good mechanical strength, and high dielectric performance for transformers, motors, and cables.

Q2: What temperature class does kraft insulation paper belong to?
A2: In conventional oil‑paper insulation systems, kraft paper is typically used within a Class A (105 °C) insulation system as described in transformer standards such as IEC 60076‑2 and IEC 60076‑14, although local hot‑spot temperatures may be higher depending on design.

Q3: How is the long‑term aging of kraft paper evaluated?
A3: Long‑term aging is characterized by measuring the degree of polymerization (DPv) of the cellulose, using viscometric methods standardized in IEC 60450; decreasing DPv indicates degradation of mechanical properties and remaining life of the insulation.

Q4: What typical dielectric strength can be expected?
A4: Technical data sheets for electrical grade kraft insulation paper commonly specify electric strength in air around 7–8 kV/mm or higher, depending on thickness, test method, and conditioning.

Q5: Is kraft insulation paper compatible with transformer oil?
A5: Yes, electrical kraft paper is designed to work in transformer oil or other insulating liquids, offering high oil absorption, stable dielectric behavior, and predictable aging characteristics in oil‑paper insulation systems.