What Is Kraft Paper?
Kraft paper is a high-strength brown paper produced from wood pulp using the kraft (sulfate) process. In cement bag manufacturing, kraft paper is the primary material used to create multi-layer valve sacks that protect cement during transport, storage, and handling.
The word “kraft” comes from the German/Swedish word for “strength” — and strength is exactly why this paper dominates industrial packaging. Kraft paper’s long fibers give it exceptional tear resistance, burst strength, and tensile strength compared to other paper types.
Why Kraft Paper for Cement Bags?
Cement bags must withstand:
- 50kg of weight pressing on the paper walls
- Drops from 1–2 meters during handling
- Stacking pressure of 10+ bags high
- Abrasion from rough surfaces during transport
- Moisture in storage environments
Kraft paper’s combination of strength, flexibility, and porosity (allowing trapped air to escape during filling) makes it the ideal material.
Multi-Layer Kraft Paper Bag — Cross Section
Kraft Paper Grades for Cement Bags
By Weight (GSM)
| Grade | GSM Range | Layers Used | Application |
|---|---|---|---|
| Light | 60–70 GSM | Inner layers | Third/fourth layer in multi-ply bags |
| Standard | 70–80 GSM | All layers | Standard cement bags (most common) |
| Heavy | 80–90 GSM | Outer layer | Abrasion-resistant outer ply |
| Super Heavy | 90–110 GSM | Single or double layer | Heavy-duty, export bags |
By Pulp Type
- Virgin Kraft — Made from fresh wood pulp; highest strength, most expensive ($550–$850/ton)
- Recycled Kraft — Contains recycled fiber; lower strength, more eco-friendly ($400–$600/ton)
- Semi-Extensible — Specially processed for 6–8% elongation; best drop resistance ($600–$900/ton)
- Extensible (Clupak) — Micro-creped for maximum stretch; premium drop performance ($700–$1000/ton)
By Treatment
- Unbleached (Brown) — Natural brown color, highest strength, standard for cement bags
- Bleached (White) — White surface for premium printing, slightly lower strength
- Wet-Strength Treated — Chemical treatment to maintain strength when wet (for humid climates)
- PE-Coated — Polyethylene coating on one side for moisture barrier
Key Properties
| Property | Typical Range | Why It Matters |
|---|---|---|
| Basis Weight | 70–90 GSM | Determines material cost per bag |
| Tensile Strength (MD) | 5–10 kN/m | Resistance to pulling apart |
| Tear Strength (CD) | 800–2000 mN | Resistance to tearing |
| Burst Strength | 250–550 kPa | Resistance to puncture |
| Elongation | 2–8% | Ability to stretch before breaking |
| Porosity (Gurley) | 15–40 sec | Air passage during filling |
| Cobb Value | 20–40 g/m² | Water absorption (lower = better) |
MD = Machine Direction (along paper roll); CD = Cross Direction (across roll)
How Many Layers?
Standard cement bags use 2–5 layers of kraft paper. The number of layers depends on:
| Layers | Total GSM | Best For | Machine Required |
|---|---|---|---|
| 2-layer | 140–180 GSM | Light-duty bags, dry climates | Standard tuber |
| 3-layer | 210–270 GSM | Standard 50kg cement bags | Standard tuber |
| 4-layer | 280–360 GSM | Heavy-duty, export markets | 4-layer tuber |
| 5-layer | 350–450 GSM | Premium bags, PE film included | 5-layer tuber (e.g., Joyda JY-T500) |
Layer Configuration Examples
- 3-ply standard: 80 GSM outer + 70 GSM middle + 70 GSM inner = 220 GSM total
- 4-ply export: 90 GSM outer + 70 GSM × 2 middle + 80 GSM inner = 310 GSM total
- 5-ply premium: 90 GSM outer + 70 GSM × 2 middle + PE film + 80 GSM inner = 310 GSM paper + PE
Cost per Bag
| Configuration | Paper Cost/Bag | Total Material Cost/Bag |
|---|---|---|
| 2-layer (70 GSM) | $0.04–$0.06 | $0.06–$0.10 |
| 3-layer (70–80 GSM) | $0.06–$0.10 | $0.10–$0.15 |
| 4-layer (70–90 GSM) | $0.08–$0.14 | $0.12–$0.20 |
| 5-layer (with PE) | $0.10–$0.18 | $0.15–$0.25 |
Paper represents approximately 50–65% of total cement bag material cost.
Top Kraft Paper Suppliers
By Region
Europe (Premium)
- Mondi Group (Austria/South Africa) — Leading producer of extensible sack kraft
- Segezha Group (Russia) — Large-volume kraft paper producer
- Nordic Paper (Sweden) — High-quality unbleached kraft
Asia
- APP (Asia Pulp & Paper, Indonesia) — Large-scale production, competitive pricing
- Lee & Man Paper (China) — Growing presence in sack kraft market
- ITC (India) — Indian kraft paper producer
Americas
- WestRock (USA) — Multi-wall sack paper specialist
- CMPC (Chile) — Major South American kraft paper producer
How to Select Kraft Paper
Step 1: Determine Bag Requirements
- What weight will the bag carry? (25kg, 40kg, 50kg)
- How far will bags be transported?
- What climate/humidity will bags encounter?
Step 2: Choose Layer Count
Match the number of layers to your strength requirements and machine capability.
Step 3: Select Paper Grade
- Budget-conscious → Recycled kraft, 70 GSM
- Standard quality → Virgin kraft, 70–80 GSM
- Export/premium → Semi-extensible or extensible, 80–90 GSM
Step 4: Test Samples
Always request sample rolls and produce test bags before committing to large orders. Test for:
- Drop test (1.5m onto concrete)
- Stacking test (10 bags high for 24 hours)
- Moisture exposure test (24 hours at 80% humidity)
Storage and Handling
- Store kraft paper rolls in dry, covered warehouse (humidity < 65%)
- Keep rolls off the floor on pallets or racks
- FIFO (First In, First Out) inventory rotation
- Maximum storage time: 6 months for standard kraft, 3 months for PE-coated
- Protect from direct sunlight which degrades paper strength
Frequently Asked Questions
What is the standard GSM for cement bag kraft paper? 70–80 GSM per layer is the global standard. Most 50kg cement bags use 3 layers of 70–80 GSM paper, totaling 210–240 GSM.
How much kraft paper does one bag use? A standard 50kg cement valve sack (500mm × 800mm) uses approximately 80–120 grams of kraft paper, depending on layers and GSM.
Can I mix virgin and recycled kraft in multi-layer bags? Yes, this is common. Many manufacturers use virgin kraft for the outer layer (strength and printability) and recycled kraft for inner layers (cost savings). This combination offers a good balance of performance and cost.
Does kraft paper quality vary by supplier? Significantly. Paper from different suppliers can vary in fiber length, tensile strength, porosity, and consistency. Always test samples from new suppliers before switching. European and South American kraft papers generally have the highest quality.
Learn how to process kraft paper into bags in our Paper Valve Sack Manufacturing Guide or explore tuber machines in the machine directory.