Properties and general requirements for composite membranes

General properties of composite membranes

• Tensile force:  Is the force to break the material per unit area. Oriented PP or polyester films have high tensile force values ​​(≥ 400kp/cm2), cello-phane can reach 600kp/cm2 but LDPE is only from 100 - 200.
• Tear strength: Very important and affects the end use of some packaging materials. This value is a guide to the thin film's ability to withstand applications when operating certain devices. PE has high tear strength while Cellophane and polyester films have low values.

• Impact resistance: A particularly beneficial property when packaging heavy products in plastic film or in large containers that must withstand impact during transportation. This property testing method drops a mass onto a material and measures the relative force needed to penetrate or break the material.
• Hardness: In some packaging equipment using plastic films, this property can be important. But it is also important for bottles and other containers where solid packaging requires minimum wall thickness values ​​and maximum strength. Hardness values ​​can also be measured by measuring and calculating the material deflection when stretched.

• Heat resistance: Includes some of the following properties
  • Soft point: Vicat soft point: Temperature when a needle penetrates 1 mm of the test sample.
  • Flow index: is the flow rate of a thermoplastic at a given temperature under a specific pressure and through a gap of a specific size within a given period of time. Flow index represents the amount of resin flowing through the membrane in grams in 10 minutes.
  • Heat welding strength: represents the force needed to separate two heat-welded surfaces from each other in a perpendicular direction. PE has a very high heat flux and Cellophane has a much lower value. 
• Another factor to consider is whether the thermal film becomes brittle when subjected to low temperatures. 

• Humidity resistance: This is a very important factor when determining the suitability of plastic film when packaging many types of products. Some products need to be protected from moisture from the outside, others require that the inside not be allowed to evaporate through the packaging. There are several methods to determine this value, the simplest method is to stretch a membrane sample on an object containing water, then place it in a storage room containing a desiccant so that the substance absorbs the water vapor passing through. membrane. The amount of water in the container before and after the test period and the value of water vapor transmission rate (WVTR: Water Vapor Transmission Rate) or moisture transmission rate (MVTR: Moisture Vapor Transmission Rate) are expressed in terms of water content. measured in grams diffused through 1m2 (or 100in2) of membrane in 24 hours (g/m2/24h or g/100 in2/24h). 

• Gas barrier: Not the same as water vapor permeability. In this case, the transmission rate of special gases such as N ₂ , CO ₂ and especially O ₂ is determined. The method for determining gas permeability is to determine how much gas diffuses through the material in a given time period. In principle, this method is similar to the method used to determine WVTR mentioned above. The unit of this value is cm3/m2/24h or cc/100 in2/24h.
• Heat sealability The heat sealability of thermoplastics depends on the following conditions:
  • Softening temperature; temperature and pressure at the weld; heat sealing time
  • Structure of the membrane or itself
  • The ratio of crystal formation to the ratio of creating amorphous structure of the polymer
  • Amount of additives

• Surface treatment (Corona treatment) Films with low polarity (PE, PP) are often very difficult to adhere to printing ink and Surface wetting of materials depends on their surface energy. Therefore, to increase the printing properties of these materials, people often have to treat them with Corona.
• Elongation: is the amount of plastic material that will stretch before breaking. The more a material stretches, the better it can withstand impact loads and is less likely to break. This is very important, especially for heavy plastic bags. Elongation is expressed as a percentage of the original length. Elasticity is expressed as a percentage of the original length. PP and PVC have this value quite high, up to 450%, polyester and PS have very low stretch values.
• Hardness: of plastic materials is determined by the Rockwell method. Use steel balls of special diameter and weighed with different loads acting on the material. The depth of the indentation when the load is removed is measured. The higher the Rockwell value, the harder the material.
• Elasticity: Is an important factor related to flexible plastic packaging. It describes the ability of a material to return to its original shape and size after being deformed.
• Dimensional stability: in some cases can be severely affected by changes in relative humidity surrounding the packaging. Some materials stretch, others contract, while some are unaffected.
• Slippage: is the friction that occurs when the plastic film surface contacts another type of film or with a certain piece of equipment - This value can be measured by using a tilt table, recording the angle of inclination that With this value, the test sample overcomes surface friction. Slippage can be adjusted with membrane additives. For example, PS membranes have 3 types of slippage:

                         – High slippage, coefficient = 0.1 – 0.3

                         – Average slippage, coefficient = 0.3 – 0.5

                         – Low slippage, coefficient > 0.5

• Gloss and opacity: These are important properties for flexible plastic packaging because many customers require transparent materials to have a glossy and bright surface. Opacity appears as a milky color that will lower the transparency of the film. The comparison values ​​measure the penetration and reflection coefficients for the test sample.
• Flammability: Some types of flammable films such as cellophan, for example, PE burn slowly and burn into drops. PVDC is self-extinguishing but hard PVC is difficult to burn.