Sacks

Polyethylene sacks:

What is polyethylene?
Polyethylene is manufactured by the polymerisation of ethylene gas. The polymerisation of ethylene gives long chains of hydrocarbons which branch out in several directions. The level of spread determines the type of polyethylene. The use of several types of polyethylene has advantages, such as the very high fusion and freezing points. Depending on the application, the density of the plastic is decided. There are four forms of this plastic polymer that are used:

• High-density polyethylene (HDPE): it has a lower degree of branching of the chains of the polymerisation. This type of polyethylene is rigid, robust and long-lasting. It is durable and capable of resisting both high and low temperatures; it is opaque and is found in storage containers, water barrels, et cetera. It is also used in bags for the packaging of food in grain form, minerals, fertilisers, cement and other voluminous materials.
•  Low-density polyethylene (LDPE): this is generated by the polymerisation through free radicals, which gives rise to short- and long-chain branching. This type of polyethylene is very ductile, but it has lower tensile strength and durability. With lower tensile strength than HDPE but with greater flexibility and resilience, LDPE is used to manufacture packaging foam, disposable gloves and plastic wrapping.
• Linear low-density polyethylene (LLDPE): The polymerisation of this type of polyethylene gives rise to significant number of short branches. LLDPE is strong and long-lasting with higher puncture resistance. It has higher tensile strength and is longer-lasting than LDPE.
• Polyethylene terephthalate (PET): The type of plastic that we all use when we buy soft drinks or bottled water. PET is recyclable and some ecological brands use recycled PET products to manufacture fashion items such as socks or beanie hats.

All types of polyethylene have both industrial and domestic applications. Sacks can be made in high-density polyethylene, low-density polyethylene and made-to-measure in low density polyethylene, from the lightest to the heaviest.

The majority of LLDPE (linear low-density polyethylene) products are no thicker than 1.5mm, but low-density polyethylene can be up to 6mm or more in thickness. Without going technical, the main difference between the two types of packaging is that low-density polyethylene offers a greater capacity for stretching.

Polypropylene or raffia sacks (PP)

What is polypropylene?

Polypropylene, the most frequently used plastic resin in packaging materials is a thermoplastic polymer of propylene. Polypropylene is a light and long-lasting material, mechanically rugged and resistant to wear, fatigue, impacts and extreme temperatures.

The advantages of polypropylene (PP) sacks are:

• 100% reusable and long-lasting
• Easy to clean and antibacterial
• Light
• Breathable or water-proof with a laminated film
• Economical
• Mechanically rugged and split resistant
• Withstands high temperatures
• Resistant to most acids and alkalis, organic solvents and reducing agents
• Non-toxic
• Non-staining

Types of polypropylene:

There are two main types of polypropylene:

1. Homopolymers: A polymer composed of one type of monomers.
2. Copolymers: a high molecular weight chemical compound formed by the molecules of two or more different compounds (monomers).

To compare polypropylene sacks, check how they are made (heat-sealed edges or sewn, the latter being much stronger) and in the grams per square inch (GSI). The higher the GSI, the thicker and stronger the sack will be.

 What is the difference between woven and non-woven polypropylene sacks?

The packaging material world is incredibly diverse and the enormous variety of PP sacks can be confusing when trying to choose the right one for a particular need. Although all polypropylene sacks are made from the same plastic resin, they are manufactured differently. Woven polypropylene is composed of PP plastic threads which are woven together, creating a long-lasting strong and flexible material. In non-woven polypropylene the plastic fibres are fused together with heat to create a sealed sheet. Both processes produce long-lasting reliable sacks for all types of packing applications.

The woven or raffia PP tends to fray at the edges, although experienced polypropylene manufacturers take measures to avoid fraying and to guarantee the structural integrity of the sack.

Can polypropylene be recycled?

Yes. Polypropylene is recycled by melting at very high temperatures to eliminate contaminants. Recycled polypropylene can be mixed with virgin material up to 50%. Polypropylene is the plastic with the least impact on the environment.

PP sacks are an ecological and reusable option due to their excellent resistance to mechanical damage and all types of potentially damaging elements such as organic solvents, degreasing agents, acids, alkalis and others.

Kraft paper sacks

Kraft paper bags are perhaps the most common paper bags in our daily life. When buying in supermarkets or clothes shops the client will usually have paper bags at his or her disposal. But kraft paper is also used industrially.

What is the kraft process?

The kraft process is a chemical method for converting wood into pure cellulose fibre pulp. The process is used to separate the fibres from the lignin that bonds them. The cellulose fibres produced are very resistant and are sometimes called kraft fibres. Manufacture is normally from soft woods. The long fibres provide resistance to the paper and chemical products are added while wet to further improve resistance. The paper is manufactured in white and brown.

Area of application

For wrapping products, it is used in the food, agriculture, chemical among other industries. Kraft sacks are ideal for powder products such as plaster, cement, clays… that require high quality paper. They are also used to transport food products such as flour, sugar, starch, additives, dry or processed fruit, etc.

Used mainly as open or valve sacks, kraft paper sacks destined for food use have two or three layers of paper and are often combined with a tube made of film without polyethylene (PE) inside and/or with a paper cover.  The tensile energy absorption (TEA) values of the paper for sacks have improved over the last decade through intense research and development programmes.

Are Kraft paper sacks ecological?

Not only are they not toxic, they produce few carbon emissions and respect the environment by not contaminating. In line with environmental standards, it is an internationally recognised ecological material. The growing awareness of governments and ecologists over the dangers of the use of plastics has obliged them to focus on more ecological alternatives. This has favoured the adoption of kraft paper sacks. Recycling continues to be a major global tendency. In Europe, the demand for kraft paper for sacks has grown due to a greater awareness of the need to protect the environment.