自強之路

One of the oldest and most popular forms of packaging are bags.1 They are used for food storage, packaging and transporting goods and for waste disposal among many other uses. A plastic bag or polybag is defined as a container made of a thin, flexible plastic film which is open at one end. Although in any size, it is often called a "sack" if it can hold 25 kg / 50 lb or more. They are made of a multitude of materials. However, paper and polyolefin bags are undoubtedly the most popular types of bags and the lowest in unit cost. The majority of these plastic bags are heat sealed whereas paper bags are usually bonded and sealed with adhesives. 
HIGH-DENSITY POLYETHYLENE (HDPE)
Bags are often made of high density polyethylene (HDPE). This plastic is usually milky white or semi-translucent, has superior puncture, low permeability and temperature resistance, and is more rigid and stronger than LDPE / LLDPE but also tears easier and has a propensity to crinkle. Most shopping, grocery and trash bags in the USA are made of this plastic. HDPE is often chosen for its high strength and low cost.  
MEDIUM-DENSITY POLYETHYLENE  (MDPE)
Medium-density polyethylene is less opaque than HDPE but not as clear as LDPE. Bags made of this plastic are usually stronger than LDPE but do not stretch as much. This type of polyethylene is used on a much smaller scale than HDPE and is often used for garbage bags and for consumer packaging.   
LOW-DENSITY POLYETHYLENE (LDPE)
Low density polyethylene or LDPE is another very popular polyolefin. Bags made from this material are either white in color or clear. LDPE is often chosen for consumer packaging where strength is not a requirement like bags for bread, newspapers, frozen food, fresh produce, dry cleaning garments, etc. Bags made from this material are very flexible and easy to stretch, and are clear and glossy. They also have good tear resistance and good moisture barrier properties but only fair gas barrier properties. 
LINEAR LOW-DENSITY POLYETHYLENE (LLDPE)
Linear low-density polyethylene has similar properties as LDPE and finds uses for similar applications. However, there are some differences which makes LLDPE preferable for different applications. For example, LLDPE is selected for its higher tensile and impact strength and better heat sealability, whereas LDPE is often selected for its higher clarity, ease of processing and higher gloss. Important applications include shopping, frozen food, garbage, news paper, and general food bags. 
POLYPROPYLENE (PP)
Polypropylene (PP) is a thermoplastic of high clarity, high gloss, good tensile strength, excellent chemical and heat resistance. It has a higher melting point than PE which makes it suitable for applications that require sterilization at high temperatures. Like PVC it has low gas permeabiltiy, i.e. it is not breathable, and is easy to heat seal. PP is often chosen for food packaging where low permeability, heat sealing and high clarity is advantageous or required like bags for candies, cookies, herbs, nuts and other confectionaries. 
DEGRADABLE OLEFINIC BAGS
Degradable bags (also known as oxo-degradable or UV degradable bags) have lately gained a lot of attention. These plastic bags are often based on degradable polyethylene which contains additives (organic transition metal compounds) and/or degradable monomers that cause the plastic to slowly break down when exposed to sunlight, moisture, and oxygen. However, these bags are not neccessarily biodegradable under composting conditions since the plastic fragments might not decompose into carbon dioxide and water or the decomposition is too slow to be considered compostation.3 
BIOBASED, BIODEGRADABLE AND COMPOSTABLE BAGS
Biobased bags are used on a much smaller scale than PE bags as they are more expensive and less strong for same gauge of material. They are made from renewable crop-derived materials, mainly corn starch and biobased aliphatic polyesters like polylactic acid (PLA) and polyglycolic acid (PGA). These plastics are fully biodegradable and compostable and considered not harmful to the environment. 
The market share of biodegradable and biobased bags is rather small but is poised to grow. Starch-based bags are ideal for lawn, leave, dog-waste and food scraps collection for the purpose of composting but are also used as shopping, produce and retail bags. 
REUSABLE TEXTILE AND PLASTIC BAGS
Reusable bags are very popular in many European countries. They are often used as grocery shopping bags. They are either made of high-quality plastics or of natural or synthetic textiles like polyester, cotton or jute. These materials can be reprocessed at the end of their useful life but require more material and energy to manufacture. 
PAPER BAGS
Paper bags are very popular in the U.S. and are mostly used as grocery shopping bags. Since they are made from plants (cellulose), they are fully biodegradable, compostable and recycable. However, the production of paper bags requires significant amounts of water, chemicals and more energy to manufacture than plastic bags. Also, paper bags are usually not reusable.

1It was estimated that at least 500 billion carrier bags were produced worldwide2 with 100 billion in the EU alone3.
2National Geographic News, September 2, 2003
3European Commision, Brusssels Press Release, November 4, 2013
4The terms biodegradation, biodegradable and compostability are often misused. Important national and international standards for testing biodegradation and compostation include the US standard ASTM D6400, ASTM D6868 and the Europena Standard EN 13424.