Within the FIBC industry, there are a vast number of bulk bags to choose from and even more terms used to describe them. With so many options available, it may be beneficial to understand the basics of a bag and the jargon that you may encounter.
Bulk bags are used to store and transport various types of dry, flowable products. Due to the design and construction of bulk bags, where woven polypropylene fabric is cut and sewn together, the product contained inside the bags is not 100% protected from the elements. Typically, they are constructed using either coated or uncoated woven polypropylene fabric. Bulk bags manufactured from uncoated woven polypropylene fabric are ideal for products requiring breathability as the fabric allows for air flow. When tested, the smallest particle size retained during a filtration process is 120 microns.
A polypropylene coating, approximately 1 mil thick, can be added to one side of the woven polypropylene fabric to further reduce breathability, as some products are more sensitive to air exposure. Coated fabric can also be utilized in the construction of bulk bags intended to hold products with small particle sizes.
For products susceptible to chemical reactions, you have the option of using liners inside bulk bags to attain the necessary barrier properties. The two most commonly sought out barrier properties are the Water Vapor Transmission Rate (WVTR) and the Oxygen Transmission Rate (OTR).
The WVTR, also commonly known as the Moisture Vapor Transmission Rate (MVTR), is the measurement of the passage of water vapor through a substance. The MVTR is commonly reported in g/100in2/day. For products that are hygroscopic, or water absorbent, a bulk bag liner with a good MVTR is needed and the closer the rating is to zero, the better. This will ensure dry products stay dry and moist products stay moist.
The Oxygen Transmission Rate (OTR) is the measurement of the amount of oxygen gas that passes through a substance at a specified temperature and relative humidity. The OTR is commonly reported in cc/100in2/24hr. The OTR of the liner material should be considered for products sensitive to oxygen exposure. Although the amount of oxygen in the air we breathe is only about 20%, it is a very reactive element and causes spoilage in food products. A ‘high oxygen barrier’ product is one with an OTR of less than 1.
High temperatures can also affect the performance of bulk bags and liners. Heat exposure of more than 70°F will cause the polypropylene molecules in the fabric tapes to shift and loosen. As the temperature increases, so does the movement of the molecules. The tensile strength, or the amount of force needed to pull to the breaking point, of the fabric is reduced as the molecules lengthen. This results in potential stretching of the woven polypropylene, thus weakening the bulk bag.
BAG Corp strongly advises against filling a bulk bag with product at temperatures of more than 200°F.
The standard bulk bag liner is manufactured from polyethylene resin that is melted and then extruded into a blown film. It is important to note that liner material will begin to melt to a molten state between 260°F and 295°F. Similar to the woven fabric, high temperatures allow the plastic molecules to soften and shift. Polyethylene liner material will begin to soften between 195°F and 210°F.
BAG Corp strongly advises against filling a ‘lined’ bulk bag with product more than 170°F.
To help select the correct bulk bag for your application, you can begin by considering the physical and chemical properties of the product intended for the bulk bag.
- To determine the necessary volume of your bulk bag, you’ll need to know the bulk density of your product and how much weight you expect to ship per bulk bag.
- Flow characteristics of your product should be considered when choosing the bottom and discharge option of the bag.
- Filling machine variables may affect the design of the top of the bag. A duffle top will allow a larger range of machines to fill the bag, while a spout top bag may be the best solution if you’re using machinery with a single sized output.
- Particle size of the product should be considered when determining the fabric in the bulk bag construction.
- Polypropylene coatings can be added to one side of the fabric to reduce the chance of products sifting.
- For sensitive bulk powders, or filling and discharging where flammable gasses are present, static protection is essential to prevent a fire or catastrophic explosion. For these applications, Type C or Type D electrostatic FIBCs offer static protection.
- Instances where the product is a hazardous material and has a UN number assigned to it, a hazmat bulk bag with UN construction is required.
- When it comes to the efficient handling of a bulk bag, various strap or lifting loop options exist. Typically, four corner lifting loops are located on the bulk bag. ‘Cross-corner’ loops may be beneficial to those who lift bulk bags with a forklift.
- If the bulk bags are to be stored outdoors, it is vital that the fabric contains a UV inhibitor to prevent the rapid degradation due to the harmful effects of UV rays. BAG Corp recommends that bulk bags are not stored in direct sunlight.
- If storage or shipping space is limited, a baffled bag may be used. When filled, they maintain their rectangular shape and may be triple-stacked, depending on height, weight, and product stability.
BAG Corp offers stock bulk bags through our online store and available for immediate shipment from our warehouse. If you have questions regarding your product or which bulk bag meets your needs, contact one of our BAG Corp Customer Product Specialist for assistance.
