Stretch film (Stretch wrap film) is also called elastic film. It can be used to Stretch and wrap the goods tightly by the viscoelasticity of the film’s surface and the tension used during use to protect goods.
Stretch film can be used for transport packagings, such as large pallets of winding packaging, containers, and boxes of winding packaging, can also be used for short-distance transport and sales packaging, such as commodity packaging, the winding packaging of stainless steel pipe, plastic pipe and profile, plastic decorative board, winding floor, winding carpet, etc.
The method of use can be divided into machine-used stretch film and hand-used stretch winding film, most of the stretch winding film is transparent, and a small number of black machine-used stretch film is mainly suitable for more expensive products, being unnoticed can effectively prevent theft.
PVC (polyvinyl chloride), LDPE (Low-density polyethylene), LLDPE (linear low-density polyethylene), EVA (ethylene-vinyl acetate copolymer) and PB (polybutadiene) are the raw materials used for the stretch winding films, at present, LLDPE and LDPE are mainly used in the production of stretch film in China.
According to the production equipment, there are two main types of stretch winding film production process: cast line and blown line. Among them, the application of the earliest is blow molding film method, the use of salivating film method than blow molding film method, high production efficiency, good product thickness uniformity, high transparency, and can reduce the amount of tackifier, in the production of stretch film, the salivation film-forming method is in the dominant position at present, and the blow molding process is mainly used in some special places, such as grass film, high puncture resistance film and anti-rust stretch film. However, tape casting also has the disadvantage of a large one-time investment.
Due to the low investment threshold, the main force of the domestic market for stretch film is some small manufacturers using domestic equipment, they account for about 70% of the market share.
Tackifier for stretch film
| 1) the function and type of tackifier
There are two basic properties of wrapping film for packaging, one is good mechanical strength, good extensibility and good resilience, the other is its surface (single or double-sided) to have strong viscosity. Polyethylene can be used to make films with good mechanical strength, good extensibility and good resilience, but it is difficult to make plastic films with sufficient surface viscosity, it is often necessary to increase the surface viscosity of the film by adding tackifier additives.
In industrial production, low-molecular-weight polyisobutylene (Pib), Low-density polyethylene density below 0.905(VLDPE) and Eva resins can be used as tackifiers for polyethylene stretch films. Eva resins are rarely used due to their potential for sour taste, high price and low cost performance, while Low-density polyethylene resins are characterized by good softness, high viscosity and high puncture resistance, the low molecular weight polyisobutene can be used as the tackifier of polyethylene stretch film, but it has the problem of low performance-price ratio and is seldom used in practice, the utility model has the advantages of larger viscous adjustment range, convenient viscous adjustment, lower price, and obvious comprehensive advantages of performance and price. Therefore, low molecular weight polyisobutylene and its masterbatch are the most commonly used tackifier in the production of polyethylene stretch film at present.
The reason why polyisobutene can be an excellent tackifier is not only because of its inherent viscosity but also because it is incompatible with polyethylene film, polyisobutylene will gradually migrate to the surface of the film, resulting in viscosity.
| 2) the performance requirements of the stretch film for polyisobutene
The“Stickiness” required in the use of polyethylene stretch film actually requires both good peel stickiness and good lap stickiness. The higher the peel viscosity, the more difficult it to lose the tray, and the better the film sealing; the higher the lap viscosity, the film is less prone to longitudinal displacement, so in order to obtain a strong packaging, in particular, such as pallets of large-scale integrated packaging of goods, must make the film peel viscosity and lap viscosity to achieve equilibrium.
Low molecular weight (MW) polyisobutylene (LMP) plays a decisive role in the adhesion of polyethylene film. Different molecular weight results in different adhesive properties. The high molecular weight of polyisobutene makes the film have higher final peel adhesion, but with the increase of the molecular weight of polyisobutene, the bonding adhesion will decrease And the relatively low molecular weight of polyisobutene has a good bonding adhesion, but the molecular weight is too low because it is more volatile will lead to the retention of viscosity is too short. Therefore, the optimum performance of the stretch film can be obtained by selecting the suitable molecular weight of polyisobutene. The results show that the polyisobutene with an average molecular weight of about 1300 is more suitable for increasing the self-adhesive property of the stretch film.
In order to achieve an ideal balance between peel viscosity and lap viscosity, some manufacturers also choose to combine different molecular weights of polyisobutylene to increase the viscosity of polyethylene stretch film. For example, polyisobutene, a blend of low-molecular-weight polyisobutene with different molecular weights, is provided by the French company, polyide, for tackifying polyethylene stretch films.
| 3) the optimum addition of polyisobutene
The self-adhesive property of the stretch film is positively correlated with the amount of polyisobutene. However, when the amount of polyisobutene exceeds 3%, the increase of the amount of polyisobutene increases, the self-adhesive property of the winding film becomes less obvious, and if the amount of polyisobutene is increased, the precipitated polyisobutene will agglomerate on the surface of the film, resulting in uneven self-adhesion of the film, and even difficult to open the film coil. In order to ensure the final performance of the stretch film, and to take into account the cost of raw materials, the amount of polyisobutene should be controlled between 1% -3%.
PIB has an exudation process, which usually takes about three days before the film surface is sticky enough, the migration speed of PIB is related to the temperature of storage: the migration speed of PIB is faster in summer and slower in winter. Therefore, the proportion of polyisobutene should be adjusted with the seasonal change, that is, the amount of polyisobutene added in winter production is a little more than the amount of polybutene added in summer.
The amount of polyisobutene used in the different production processes is different because of the difference in equipment. Because the uniformity of film thickness produced by the blow molding process is not as good as that of the tape casting process, the amount of polyisobutene used in the blow molding process should be slightly higher than that in the tape casting process, the specific use of the difference also need to refer to different users of the specific process conditions.
| 4) Pib viscidity masterbatch
Low molecular weight polyisobutene (LMP) is a viscous liquid substance. It is a mixture of PIB and polyethylene particles. When a general purpose extruder is used to produce the plastic film, the material can not be extruded smoothly. Special extrusion equipment is required, an extruder with a forced feeder, such as a hopper, is forced to feed a mixture of LLDPE and PIB, or a special extruder is used to pump a viscous liquid into the PIB. For the convenience of production, high concentration PIB tackifying masterbatch, which is made by mixing PIB and polyethylene in a certain proportion, has been developed for the production of polyethylene stretch film, it and plastic products often used in the production of a variety of masterbatch similar to the shape has good mobility, easy to use, can use ordinary film molding unit, the smooth production of polyethylene stretch film.
| 5) comparison of tackifying Masterbatch and polyisobutene
As the viscosity masterbatch is also composed of polyisobutylene, so as a viscosity agent in the performance and polyisobutylene is no different, the greatest advantage of the viscosity master batch is easy to use.
At present, there are two kinds of tackifying masterbatch on the market: domestic and imported, but the price is higher than that of polyisobutene because of the cost in the production process of tackifying masterbatch, as a result, the formulation cost when it is used as tackifier for polyethylene stretch film is obviously higher than that when it is used directly as tackifier. As a result, its market share is far less than polyisobutene.
