Progress and Application of PVA and Starch Films

Water-soluble films are made of water-soluble polymers. Water-soluble polymers can be divided into three major categories: (1) synthetic polymers such as polyacrylamides, polyacrylic acids, and polyvinyl alcohol polymers; (2) semi-synthetic polymers such as cellulose ethers, starch, and natural Derivatives of gums, etc.; (3) Natural polymers such as natural starches and natural gums. In theory, all three types of polymers can be made into thin films by different methods such as casting, coating, spraying, and melt extrusion. This article only discusses melt-extruded PVA and starch soluble films, as melt extrusion is the most common and most efficient method for industrial production of films.

PVA is a water-soluble polymer with excellent properties. Water-soluble films made of PVA have excellent mechanical properties, excellent gas barrier properties, and are biodegradable under certain conditions. Industrialized water-soluble films were first produced by PVA using the cast method. One of the difficulties in production using melt extrusion is that PVA molecules contain a large number of hydroxyl groups, form hydrogen bonds, and the molecules contain a crystalline structure that makes the melting point higher than the decomposition temperature. The melting temperature of PVA is about 190°C and the decomposition temperature starts from 140°C. In the processing process, plasticizers (such as glycerol and water) are first used to gel the PVA at a certain temperature to weaken the hydrogen bonds between the polymer chains and break the crystal structure to reduce the melting temperature.

The use of polymer materials derived from renewable resources to research and develop environmentally friendly polymer materials is a hot research area in the world today. Due to its wide range of sources and low prices, starch is considered to be the most promising material. Like PVA, starch molecules also contain a large number of hydroxyl groups, but also contains a crystalline structure, melting point (230 °C) is much higher than its decomposition temperature (170 °C). The problem of destroying the crystal structure therein is also involved in the processing, which is the gelatinization of the starch.

This article describes the latest results and application of PVA and starch soluble films, especially the latter.

1 PVA water soluble film
In the late 1960s, Takigawa et al. first reported the production of PVA films by melt extrusion. Melt-extruded PVA film was originally made by a two-step process. The first step is to mix PVA with a plasticizer and squeeze it out from the extruder; the second step is blown film or melt extrusion and drawing. membrane. Later, in order to reduce costs and improve production efficiency and product quality, a one-step melt extrusion process for the production of PVA films was developed.
1.1 One-step melt extrusion processing of PVA film
Figure 1 (simply) is a flow chart of a one-step melt extrusion process. PVA is first treated with a plasticizer in a high-speed kneader to produce free-flowing gel particles, which are then melt-extruded into a blown film or extruded to form a film. In this process, it is critical to use a high-speed kneader to make free-flowing gel particles.

In the high-speed kneader process, two goals are achieved: first, gelling PVA, destroying the crystal structure of PVA to reduce its melting temperature; second, it is necessary to keep PVA in a flowable particle state so that it can be squeezed. The outflow hopper is free to flow in order to feed. To achieve these two goals, PVA was first placed in a high speed kneader and the plasticizer slowly added at a low speed (500 r/min). At this point, the plasticizer and PVA were uniformly mixed and adsorbed on the surface of the PVA particles. Then gradually warmed to 80 °C, and increase the speed to 2000r/min, so that the plasticizer penetrated into the PVA particles to swell. In this process, the crystal structure in the PVA particles is broken due to the penetration of the plasticizer and the force of the particle swelling process, causing the PVA to gel. The result is similar to the first extrusion granulation in the two-step extrusion process. At this point, due to the gelation of PVA and the higher temperature, the particles increase several times and become soft, tending to agglomerate. However, the PVA particles remain free flowing under the high speed shear of the high speed kneader.

The treated PVA can be directly used for melt extrusion blown film or extrusion stretched film. Since it is a one-step extrusion method, not only the processing cost is reduced, but also the water content as a plasticizer is easier to control in the gelation process.

1.2 Water solubility of PVA film
Due to different product requirements, PVA water-soluble films can be made into products with different solubility and dissolution rates. The solubility and dissolution rate of PVA films are determined by many factors. For example, the degree of polymerization of PVA, the extent of alcoholysis, and the type and content of plasticizers, and the like. In particular, the type and content of plasticizers are commonly used to control the solubility and dissolution rate of PVA films. In theory, all small molecules with hydroxyl groups can be used as plasticizers for PVA, such as various alcohols. The most commonly used in the industry is a blend of water and glycerine (glycerin). Figure 2 (slightly) is a melt-extruded blown film of polyvinyl alcohol (ELVANOL 71-30) from DuPont, with different proportions of water and glycerol as plasticizers, with different plasticizer content. The time required for complete dissolution in water. It can be seen that the PVA film is more soluble in water as the plasticizer content increases. In mixed plasticizers of water and glycerol, the film containing a higher glycerin ratio is more soluble in water. This is mainly because glycerol has a strong water absorption and less plasticizer loss (evaporation) during processing.

2 starch-based water-soluble film
Starch-based materials are difficult to process and shape; this is due to the complexity of the microstructure, that is, one can design the microstructure of the polymer, control the relative molecular mass and its distribution, but it is difficult to change the internal structure of the starch granules. The development of the internal structure of starch granules is to meet the needs of the plant itself, such as storing energy, and it is rather complicated. Starch is a multi-mineral substance composed of repeating glucosyl units. The structure of the starch contained therein is also different depending on the plant species and genetic background. Chemically speaking, most natural granular starches are mixtures of the following two types of starch: one is a straight-chain structure containing alpha glucosyl units linked by 1,4 glycosidic bonds, ie amylose; the other is branched chain Starch, containing hyperbranched structures with 1,6 glycoside chain short branches. From a physical point of view, starch granules contain crystalline and amorphous structures.

The same with PVA, starch also contains a large number of hydroxyl, also contains a crystalline structure (about 30%), the melting point is much higher than its decomposition temperature, so in the processing process also have to destroy the crystal structure, that is, starch Paste. The gelatinization properties of different starches vary widely.

2.1 Paste and Processing of Starch
The processing properties of starch-based materials are controlled by the starch gelatinization process. The so-called gelatinization means that by introducing small molecules into the glycan chains and destroying the crystals thereof, the starch particles become amorphous structures. In theory, all small polar molecules can be used as gelatinizers. In practical applications, water is the most widely used gelatinizer. Water has been used in the starch food industry for many centuries.

DSC analysis is widely used in gelatinization studies of starch. The gelatinization of the starch begins with the amorphous part. In the presence of water, the orderly to disordered conversion of starch can be carried out in various ways. The occurrence of the gelatinization process mainly depends on the content of water in the starch, under the action of no shear stress, such as in starch. When the moisture content is less than 70%, gelatinization takes place in two steps, namely gelatinization of the amorphous phase and melting of the crystalline phase. The melting temperature of the starch also depends on the content of moisture therein, the melting temperature decreases as the water content of the starch increases, and when the moisture content exceeds 70%, the melting of the crystalline phase overlaps with the gelatinization temperature of the amorphous phase. Figure 3 (omitted) is the gelatinization temperature for different starches at a moisture content of 70%. It can be seen that the gelatinization temperature of many starches is distributed in the range of 50-100°C. Usually, the higher the amylose content, the higher the gelatinization temperature and the wider gelatinization temperature range. One-step and two-step melt extrusion methods can be used for the production of starch-based water-soluble films.

2.2 Mechanical properties of the film
Due to the complex structure and poor homogeneity of starch, the mechanical properties of starch-based materials are affected by many factors. Examples include the type of starch, the ratio of amylose to amylopectin in starch, the relative molecular mass of starch, and the content of other substances (such as protein and fat), the type and amount of plasticizers used in processing, processing methods, and test environments. (Humidity and temperature) and recrystallization factors of starch. Here, the mechanical properties of corn starch with different linear/branched ratios at different degrees of orientation are taken as examples to discuss the relationship between the microstructure and mechanical properties of starch-based materials, as shown in Figure 4 (omitted). The degree of orientation is controlled by the three-roller speed before the sheet die at the time of melt extrusion drawing. At a traction speed of 100 mm/min, the thickness of the test specimen was about 0.15 mm, which was taken from the longitudinal and transverse directions of extrusion stretching on different samples, respectively. As can be seen from Figure 4, (1) high amylose starch has strong tensile strength; (2) increased orientation can increase tensile strength; (3) orientation has a greater influence on the tensile strength of high amylose starch; (4) ) After the high amylose starch is oriented in tension, the difference between the longitudinal tensile strength and the transverse tensile strength increases.

Tensile strength has been widely used in various plastic products through the process of molecular orientation to improve tensile strength, such as polyolefin biaxially stretched film, tensile strength can be increased by more than 10 times, but the improvement of starch-based plastic is not as obvious as polyolefin Especially high amylopectin. This is determined by the special structure of starch. As mentioned earlier, amylopectin is a hyperbranched structure with short branches. These short branches consist of only 5-6 glucosyl units on average, and each two short branches form a double helix crystal structure. The starch gelatinization process destroys the crystal structure of these double helices. A large number of short branches in each main chain are grouped together to form "microspheres", and gelatinized starch molecules are aggregated together to form a "gelball" structure. In the orientation shear