FABRICATION OF PLASTICBAGS

Catalina Amengual i Gar

Cristina Borrs Marqus

Bernat Darn Sellabona

Claudia Villar Egea

BACKGROUND

1965

Three employees of a Swedish company design and patent for "tubing for packaging purposes:

Plastic tube sealed at one side and allow the top side for the packaging of goods.

Also create handles for convenient carrying:

T-shirt plastic bag

T-SHIRT PLASTIC BAG

1975

The first plastic bags were introduced at retail stores

Plastic bags were better solution than paper bags:

Oil cheaper than wood

Less energy and water during production

Take up less space

More durable

Can hold more weight

Cost about 1/3 of a paper bags

NOWADAYS

About a trillion plastic bags are used every year.

Depends on the country environmental-friendly laws that regulate its use.

PRODUCT

PLASTIC BAGS

Day-to-day object

Used to transport and/or preserve small amount of goods

Most common form of packaging

Made of thin, plastic film

Lightweight and flexible

Thickness between 18 to 30 micrometers.

PLASTIC BAGS

Types according to their function:

Transporting goods T-shirt BagEconomicVery little amounts of materialUsually made of high density polyethylene which can carry up to 12 kilos

Keep foods protected against moisture Highly hygroscopic bagsHumidity absorbentsMade with polypropylene laminations

Medical purposesBeing manufactured under sterile conditionsNon-porous (isolating infectious body fluids)Less expensive than other re-usable options (such as glass)

PLASTIC BAGS, EXAMPLES

T-shirt Plastic Bag Retail Handle Bag Clear Colored Gift Bag

Market Bags With Block

Garbage Bag

Bag Roll

PLASTIC BAGS, EXAMPLES

Zipper Pouch BagStatic Shielding Bag

Cone BagMicro-Perforated Bread Bag Gusset Bags

Flap Seal Bag

THE MATERIAL

PROPERTIES

Flexible

Stretchable

Stiff

Low MFI High viscosity

Long and ramified molecular chains (prevents melt fracture, better mechanical properties)

COMMON POLYMERS

High Density Polyethylene (HDPE):

Easy to process, low cost

Good low temperature toughness

Great thermal resistance

Flexible

Weatherproof

Recyclable but not biodegradable.

COMMON POLYMERS

Low Density Polyethylene (LDPE) and Linear Low Density Polyethylene (LLDPE):

Very tough

Not good low temperature toughness

Good thermal resistance

Semi-rigid

Not biodegradable.

COMMON POLYMERS

Polypropylene (PP):

Difficult to process

Tough

Good dimensional stability at high temperatures

Rigid

Not biodegradable.

MANUFACTURING PROCESS

PLASTIC BAGS

The manufacturing process can vaty, but in general it includes severalmain steps:

Mixing stage

Blown film extrusion

Printing

Cutting and sealing

MIXING STAGE

The plastic polymer is transformed (adding additives) to improve their basic

mechanical, physical and chemical properties to obtain the requirement

commercial product.

Additives are combined to create the desire properties:

Blends of clarity

Strength

Stretchability

Sealability scruff and tear resistance

UV protection

Bacterial protection

Surface appearance

BLOWN-FILM EXTRUSION PROCESS

Blown film extrusion is the most important manufacturing process to

make plastic bags.

In this stage, tubular film is extruded vertically upwards. Air is

introduced inside of the tube, and as a result, the tube expands to a

bubble with a diameter large than the diameter of the die.

When the bubble is cooled sufficiently, it is flatten and pulled

through a set of nip rolls.

The layflat is guided over several idler pulleys (rodillos tensores) to

the winder (bobinadora) where it is rolled up over core.

BLOWN-FILM EXTRUSION PROCESS

Schematic of blown film process line.

https://www.youtube.com/watch?v=8CfL5xl2N1Q

BLOWN-FILM EXTRUSION PROCESS

The blown film extrusion is the most important manufacturing process to

make plastic bags, and hence it is important to analyze basic aspects

of the process, to know the effect that they will have on the final

product.

Important parameters:

Cooling rate

Bubble stability

Film speed (screw rotation, calendaring rolls velocity)

Temperature of the melted plastic (high viscosity polymers)

Air pressure (bubble expansion)

IMPORTANT PARAMETERS

Frost line height is the point where the film changes from the

melt to the solid state with semi crystalline polymers. Film

changes from a transparent amorphous state to a hazy or

translucent crystalline state.

Dd and DB are the die and the bubble diameters.

To and Tf are the initial and final film thickness. The film

thickness is dependent on the bubble size and the draw ratio.

Vo and Vf are the die exit and final film velocity. Higher draw

ratios result in relatively higher Vf values.

MD and TD are the machine and transversal direction

orientation. (TD is also called the cross machine direction and

is designated as CD). This is an important parameter to

obtain the required mechanical properties of the bag,

especially for parameters that are closer to improve the

capacity of the bag to be loaded. The property ratios

depend on the bubble size and draw ratio. Larger bubble size

provides more transverse direction orientation and higher

draw ratios provide more machine direction orientation.

IMPORTANT PARAMETERS

Blown-up ratio (BUR) is defined as the bubble diameter divided by

the die diameter. It measures how large the bubble has expanded in

the transverse or cross-machine direction. The larger the blown up

ratio, the higher the transversal direction molecular orientation with

corresponding decreases in the draw down ratio.

The drawn down ratio (DDR) is the drawing taking place in the film

tower in the machine direction, and it is related to the blow-up. It is

also defined as the stretching ratio, as the relation between the linear

velocity of the stretching rolls (Vf ) and the velocity of the polymer at

the exit of the extruder die (V0)

ADVANTAGES OF THIS PROCESS

Advantages:

-Better property balance between the machine and transvers direction.

-Less scrap

-Equipment cost

-Capacity to make different widths with only one single die by

modifying the blown-up ratios.

EXTRUDING PLASTIC FILM

Printing stage

The rolls with the extruded material are introduced at the beginning of

the rotary flexographic machines, then the polymer film is forced to pass

through different rollers and cartridge holder until the film reached the

final line, when material is dried ink. Gravure-printing can also be used to

do this process stage, but is less common than the flexographic process.

Cutting and sealing

The next step in the manufacturing plastic bags is the cutting stage. The

first thing to be done is program the cutting machine with the required

parameters according to the type of bag which is produced. Product

width, high, geometry handles (is it is necessary) and pleats are fitted, in

order to obtain the final geometry.

Holes, vents, slits, perforations, handle punching are some of the different

options available in each production run.

Scheme of flexographic Printing process

CUTTING PROCESS

ENVIRONMENTAL IMPACT

LIFE CYCLE

CONTEXTUALIZATION

On average, plastic bags are used only for 25 minutes.

It takes between 100-500 years for a plastic bag to disintegrate (depending on the type of plastic)

1 million plastic bags are in use around the world every minute.

The average European uses about 500 plastic bags per year.

80% of marine litter is plastic

34 million tones of plastic carrier bags are produced in the UE each year.

THE THREE RS: REDUCE

It is a form of waste prevention: minimize the amount and toxicity of waste before it is generated.

Initiatives to reduce the consumption and use of plastic bags:

3rd of July is the Plastic Bag Free Day

Plastic bag free towns: the Spanish Government, the past March, approved a law that pretends gradually reduce the consumption of single use bags to suppress definitively in 2018.

THE THREE RS: REUSE

To reduce the consumption of plastic bags we can reuse the plastic bags that we have at home. This is a way to reduce the environmental impact.

Newer reusable plastic bags have been developed that both for minimize the amount of plastic being manufactured and maximize its use.

THE THREE RS: RECYCLE

Plastic bags are generally made of HDPE (high density polyethylene) and LDPE (low density polyethylene) and both are recyclable.

1. Workers hand-remove contaminants not meant to be recycled.

2. Bags are washed in flood tanks, removing all forms of non-metallic contaminants, and go through magnets, removing all the metallic impurities.

3. After wash the bags this are loaded into the recycling machine: https://www.youtube.com/watch?v=Q2IQWV8Q5JU

A RECYCLED MATERIAL: BIORESIN

The alternative to polyurethane-based plastic, bioresins are the result of the recycling process, and depending on the material of the original bag we can find:

Degradable resins: can continually be broken down into smaller and smaller pieces.

Compostable resins: can be used as mulch after being mixed with specific bio products at a composting plant.

There are a lot of companies that work producing bioresins, like the enterprise Bioresins.eu

DEGRADABLE PLASTIC BAGS

Plastic bags designed with changes in its chemical structure under specific environmental conditions. It can be broken down by chemical or biological processes.

Biodegradable polymers.

Compostable polymers.

Oxi-biodegradable polymers.

Photodegradable polymers.

Water-soluble polymers.

GREEN TRASH BAGS

Made from potato starch, a fully compostable polymer; without polyethylene and plasticizers.

Starch is a mixture of a linear (amylose) and a branched (amylopectin) polymer of glucose.

In a landfill environment, this organic material (leaves, grass etc.) releases methane gas a greenhouse gas contributing to global warming.

The procedure of manufacturing trash bags from potato starch is basically the same, but there are some differences: the complete process it is a little longer, because it is done with lower screw speed rotations, the blow up ratio is smaller and the film thickness is bigger.

THANKS.

CATALINA AMENGUAL I GARCRISTINA BORRS MARQUS

BERNAT DARN SELLABONACLAUDIA VILLAR EGEA