The raw material components of biodegradable bags are PLA, PBAT, starch, synthetic polymers, plasticizers, starch structure breakers and other additives. The starch used for the raw material of the biodegradable bag does not need to be dried or added water first. Synthetic polymers used for raw materials include polyvinyl alcohol (PVA), ethylene/vinyl alcohol (EVOH) copolymer, and ethylene/acrylic acid (EAA) copolymer. Vinyl alcohol has a molecular weight of 50,000 to 120,000, a melting point of 160 to 200°C, and a degree of hydrolysis of 75% to 98%. It is available in granular, flocculent or powder form.
In order to improve the biodegradability, 10%~50% (mass fraction) of modified polyvinyl alcohol is added to the formula. The modification methods are: functional alcohol groups are replaced with oxo groups (O==) or alkyl carbonyl groups (CR, R=C1~C4). The method is to react polyvinyl alcohol with hydrogen peroxide and peroxodisulfuric acid, or to polymerize Vinyl alcohol reacts with hypochlorite and acetic acid. Etherified with ethylene oxide or silane; esterified with sulfate or nitrate; esterified with aliphatic or aromatic; reacted with saturated or unsaturated aldehydes to form acetal functional groups. Ethylene/vinyl alcohol copolymer, ethylene content 10%-40% (mass fraction) [30%-45% (molar fraction)]; at 210℃, 0.216MPa conditions, the melt flow rate is 5~26g/10min; melting point 160 ~170°C. It can be modified, and the method is the same as that of polyvinyl alcohol. The content of acrylic acid in ethylene/acrylic acid copolymer is within 30% and can be modified. The method is the same as that of polyvinyl alcohol.
In order to adjust the water resistance of the raw material products of the biodegradable bag, a small amount of hydrophobic polymers, such as polyethylene, polypropylene and polystyrene, can be added in an amount of 2% to 3% (mass fraction) of the total weight of the plastic.
When the raw materials of biodegradable bags are made, the purpose of adding plasticizers is to improve the fluidity of high melting point synthetic polymers, so that starch molecules and synthetic polymer molecules can penetrate each other completely. Plasticizers mainly include: ethylene glycol, polyethylene glycol (molecular weight 200~4000), propylene glycol, polypropylene glycol, glycerin, sorbitol acetate and sorbitol ethoxylate and other aliphatic polyols and their mixtures. The plasticizer is required to have good compatibility with starch and synthetic polymers. The boiling point is higher than 150°C. The vapor pressure at ambient temperature (25°C) is lower than that of glycerin. It is soluble in water and the dosage is 5%-25% (quality Fraction) The total weight of the plastic.
In order to gelatinize starch, it is necessary to add a starch structure breaker. The starch structure destroyers used for the raw materials of biodegradable bags include water, uric acid [(NH2)2CO], alkali metal hydroxide or alkaline earth metal hydroxide. Starch is heated to above the glass transition temperature and melting point in the presence of water and plasticizer, gelatinization occurs, the structure is destroyed, and it becomes a disordered state, which can be well blended with synthetic polymers. The amount of structural breaker used for urea is 2% to 5% (mass fraction) of the total weight of the plastic.
Other additives added to the raw materials of the biodegradable bag include: boron-containing compounds (boric acid, metaboric acid and its alkali metal salts, borax and its derivatives), the purpose is to improve the compatibility of starch and synthetic polymers and the mechanical properties and transparency of products The dosage is less than 0.4% (mass fraction). Lithium chloride or sodium chloride has the same effect as boron-containing compounds, and the dosage is 0.5%~3% (mass fraction); aldehydes, acetals and ketones, which act as cross-linking agents, can be added as needed; bactericides, antiseptics Mold agents, flame retardants, herbicides, etc., can be added as needed.
The production of raw materials for biodegradable bags adopts master batch technology. The equipment adopts vented co-rotating twin-screw extruder. The starch, synthetic polymer plasticizer, starch structure breaker and other additives are mixed and granulated to make a masterbatch, which is then processed into desired products on various molding machines.