Abstract: This paper through the HDPE, LDPE and LLDPE three resin material production raw materials, molecular structure, density, crystallinity, softening point, corrosion resistance, temperature domain, mechanical properties, tensile strength, elongation at break, anti-environmental stress cracking (see Table 1) and the principle of industrial production, process and additives to analyze the scope of engineering applications of the three materials and their application characteristics and applicable engineering scope. And additives to analyze the scope of engineering applications of the three materials and their respective applications of the characteristics and scope of applicable projects.
Polyethylene (Polyethylene) is one of the five synthetic resins, is China's synthetic resins in the largest production capacity, the most imported varieties. At present, China is the world's largest importer and second largest consumer of polyethylene. Polyethylene is a monomer polymerization of ethylene, polyethylene plastic is a polyethylene resin as a substrate, adding a small amount of antioxidants, slip agent and other additives made of plastic products. Polyethylene is mainly divided into linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), high-density polyethylene (HDPE) three categories.
1 High Density Polyethylene, the English name is "High Density Polyethylene", referred to as "HDPE".
HDPE is non-toxic, tasteless, odorless, with a density of 0.940 to 0.976g/cm3, it is catalyzed by Ziegler's catalyst, the product of polymerization under low-pressure conditions, so high-density polyethylene has also become a low-pressure polyethylene.
HDPE is a kind of ethylene **** polymerization to generate high crystallinity, non-polar thermoplastic resin. The appearance of the original state HDPE is milky white, in the micro-thin cross-section is a certain degree of translucency. It has excellent resistance to most household and industrial chemicals, and it resists corrosion and dissolution by strong oxidizing agents (concentrated nitric acid), acids and alkali salts, and organic solvents (carbon tetrachloride). The polymer is non-hygroscopic and has good water and vapor resistance and can be used for moisture and seepage control.
The shortcomings are its aging resistance and environmental stress cracking is not as good as LDPE, especially thermal oxidation will reduce its performance, so high-density polyethylene in the plastic coil made of antioxidants and ultraviolet absorbers added to improve its shortcomings.
2 Low density polyethylene, the English name for "Low density polyethylene", referred to as "LDPE".
LDPE is non-toxic, tasteless, odorless, with a density of 0.910-0.940g/cm3, it is under high pressure at 100-300MPa, with oxygen or organic peroxide as a catalyst for the polymerization, also known as high-pressure polyethylene.
Low-density polyethylene in the polyethylene resin is the lightest quality varieties. Compared with high-density polyethylene, its crystallinity (55% ~ 65%) and softening point (90 ~ 100 ℃) is lower; good flexibility, extensibility, transparency, cold resistance and processability; its chemical stability is better, resistant to acids, alkalis and salts in aqueous solutions; good electrical insulation and permeability; low water absorption; easy to burn. It is softer in nature, with good extensibility, electrical insulation, chemical stability, processing performance and low temperature resistance (can withstand -70℃).
The shortcomings are its mechanical strength, moisture barrier, air barrier and solvent resistance is poor. The molecular structure is not regular enough, the crystallinity (55%-65%) is low, and the melting point of crystallization (108-126 ℃) is also low. Its mechanical strength is lower than that of high-density polyethylene, impermeability coefficient, heat resistance and resistance to sunlight aging is poor, easy to aging and decomposition in sunlight or high temperature and discoloration, resulting in a decline in performance, so low-density polyethylene made of plastic coil added antioxidants and ultraviolet absorbers to improve its deficiencies.
3 Linear low density polyethylene, English name "Linear Low density polyethylene", referred to as "LLDPE".
LLLDPE non-toxic, tasteless, odorless, density of 0.915 ~ 0.935g/cm3, is ethylene and a small amount of high-level alpha olefins (such as butene-1, hexene-1, octene-1, tetramethylpentene-1, etc.) under the action of a catalyst, by high-pressure or low-pressure polymerization of a **** polymerization, the conventional molecular structure of the LLDPE is characterized by its linear main chain The molecular structure of conventional LLDPE is characterized by its linear backbone, with few or no long chains, but containing some short chains. The absence of long branched chains makes the polymer more crystalline.
LLDPE compared to LDPE has the advantages of high strength, good toughness, rigidity, heat, cold, etc., but also has good resistance to environmental stress cracking, tear strength and other properties, and can be resistant to acids, alkalis, organic solvents and so on.
In summary, the above analysis, the three materials in different types of seepage control projects play an important role in their respective tasks. HDPE, LDPE and LLDPE three kinds of materials have a very good insulation and moisture-proof, seepage control performance, non-toxic, tasteless, odorless performance so that it is in the agriculture, aquaculture, artificial lakes, reservoirs, rivers on the application of the extremely wide range of applications, and get the Ministry of Agriculture of the Chinese Bureau of Fisheries, Shanghai Fisheries Research Institute, Fisheries Machinery and Instrument Research Institute of the Chinese Ministry of Agriculture, vigorously promote and popularize the application.
In the strong acid, strong alkali, strong oxidants and organic solvents in the media environment, HDPE and LLDPE material performance can be very good to play and use, especially HDPE in the resistance to strong acid, strong alkali, strong oxidizing properties and resistance to organic solvents is much higher than the other two materials, so the HDPE seepage prevention and anti-corrosion rolls in the chemical industry, environmental protection industry has been fully utilized.
And low-density polyethylene also has a very good resistance to acid, alkali, salt solution properties, and has good extensibility, electrical insulation, chemical stability, processing performance and low temperature resistance. So it is more widely used in agriculture, water storage and breeding, packaging, especially low temperature packaging and cable materials.
Table 1 HDPE LDPE and LLDPE material new energy comparison
Plastic name
Performance comparison
High density polyethylene HDPE
Low density polyethylene LDPE
Linear low density polyethylene LLDPE
Odor, toxicity
Non-toxic, odorless, Odorless
Non-toxic, tasteless, odorless
Non-toxic, tasteless, odorless
Density
0.940～0.976g/cm3
0.910～0.940g/cm3
0.915～0.935g/cm3
Crystallinity
85- 65%
45-65%
55-65%
Molecular structure
Contains only carbon-carbon and carbon-hydrogen bonds, which require more energy to break
Polymer molecular weight is small, which requires less energy to break
Linear structure, branched chains, short chains are fewer, which require less energy to break
Softening point
125-135°C
90-100°C
94-108°C
Mechanical Properties
High strength, good toughness, rigidity
Mechanical Strength is poor
High Strength, Good Toughness, Rigidity
Tensile Strength
High
Low
Higher
Elongation at break
Higher
Low
High
Impact strength
Higher
Low
High
Moisture-proof, waterproof performance
Good permeability to water, water vapor, air, low water absorption, with Good impermeability
Humidity barrier, poor air barrier
Good permeability to water, water vapor, air, low water absorption, good impermeability
Acid, alkali, corrosion, organic solvents resistance
Resistant to corrosion by strong oxidizing agents; resistant to corrosion by acids, alkalis and all kinds of salts; insoluble in any organic solvents and so on.
Resistant to corrosion by acids, alkalis and salt solutions, but less resistant to solvents
Resistant to acids, alkalis and organic solvents
Heat/cold resistance
Heat and cold resistance is good, at room temperature and even at -40F low temperatures so that there is excellent impact resistance, low-temperature brittleness temperature
Heat resistance is less resistant to heat, and the low-temperature brittleness temperature
Heat resistance, Good cold resistance, low temperature embrittlement temperature
Resistance to environmental stress cracking
Good
Better
Good
What is an opening agent?
Polyethylene plastic blown film, due to the formation of a vacuum between the film closed state, it is not easy to separate, difficult to use, affecting the efficiency of automatic packaging, which adds plastic opening agent, can solve the above problem.
Plastic opening agent is generally silica series of inorganic substances. Such as diatomaceous earth class, it is by the aquatic microscopic diatom cell remains accumulated from a white biochemical sedimentary rock, which contains fine pores, is a kind of debris with many different shapes, unique structure of the aggregate.
The chemical composition of diatomaceous earth is: SiO292%, Al2O33%, Fe2O31%, CaO 0.5%, MgO 0.5%, Na2O0.5%, K2O0.3%, P2O50.2%, and the amount of burn loss is 0.3%. Foreign diatomaceous earth sometimes CaO more, for about 6%, Na2O more, for about 2%, the proportion of other components is close to the domestic samples.
Diatomaceous earth appearance is loose, loose, fine, soft, lightweight white powder, oil absorption rate of 50~130 ml/100 g, refractive index of 1.48, density of 2.3 g/cm3, bulk density of 0.5 g/cm3, specific surface area of about 1~3 m2/g, PH value of about 8~9.
The opening agent with the original powder inorganic material, dispersibility Poor, poor uniformity, not as good as made into the opening agent masterbatch, the effect is better, also called anti-sticky masterbatch.
In the opening agent masterbatch, the proportion of the opening agent for 30% to 50%, carrier with low-density polyethylene (LDPE) dosage of 70% to 50%, and other lubricants, dispersants and so on.
Opening agent masterbatch generally add 1~5 parts can be.
1, preface
The development of the plastics industry at the same time, promote the development of plastic additives. From the earlier development of processing aids to application aids has been a leap. Opening smooth agent is one of the examples, in the development of its perfection. The earliest opening agent is inorganic talc, diatomaceous earth, etc.; the mid-term development of organic oleic acid amide, erucic acid amide and EBS derivatives, etc.; the current synthetic silica as an opening agent in the film is also more widely used. All of these additives are different degrees of side effects, mainly in the organic opening agent has a large number of precipitates on the surface of the film, affecting the film's printability, heat sealing and color; inorganic dispersion of the opening agent has always been the production of a difficult problem, if you add a lubricant to the formula and the organic dispersant will be the same to produce precipitates; the two of the pollution of the packaged goods is very serious, especially in the food packaging, liquid packaging, pharmaceutical packaging and other areas. Packaging, pharmaceutical packaging and other fields.
2, the principle of work
The plastic film adhesion problem is mainly due to two reasons: one is due to the film closed between the film to form a vacuum sealing state, it is not easy to separate; the other is a large number of film molding after the surface of the exposed molecular chain, in the two films after the closure of the molecular chain between the two films produced by the mutual entanglement of the molecular chain, so that it is unable to open. In fact, the cause of film opening difficulties are both **** exist, and the latter is the main reason. Early inorganic opening agent is to make the surface of the film to produce unevenness to reduce the film to ask the negative pressure to separate; late organic opening agent is in the film surface to form a layer of lubricating film, reduce the coefficient of friction of the film, so that they do not stick to each other. Both also hinder the entanglement of molecular chains. However, these opening agents have the above mentioned shortcomings.
The working principle of the new type of opening slip agent is: choose nano-sized silica powder, so that it is dispersed in the resin to micron-sized particles, which are formed by the aggregation of silica's own energy, without the addition of any auxiliary additives. This kind of particles is porous with interstices, irregular, large specific surface area of the loose particles, its diameter is Bu 2 microns, specific surface area of 550-600 square meters / g. Polymer in the process of the end of the macromolecular chain by the silica particles of the voids sucked, the particles at the same time become the nucleation center to make its crystallization. This greatly reduces the exposed molecular chain, so that when the two films are in contact without the entanglement of the macromolecular chain, thus solving the problem of the opening; also because the molecular chain is not exposed, the film in the friction of the object to reduce the adsorption force, thus increasing the smooth performance.
3, the characteristics of the new opening agent
3.1 the current opening agent problems
Inorganic opening agent to add a large amount of dispersion difficulties, affecting the transparency of the film, strength.
Organic openers have precipitates that affect the color, heat sealability, printability of the film, and contamination of the packaged goods.
3.2 Characteristics of the new type of opening agent
No volatile substances and precipitates, to ensure the quality of the packaged goods;
Improve the transparency of the film and surface finish;
The film has a reinforcing effect, to improve the creep performance;
Improve the film in the high-speed packaging line of the anti-adhesion; does not affect the film processing, printing, heat sealability;
Improved film processing, printing, heat sealability;
The film is not a good choice, but it can be used in the packaging line, and it is easy to be used in the packaging line.
Non-toxic, non-polluting, can be used in food, medicine and other industries.
4, the use of
New opening slip agent is suitable for polyolefin blow molding, casting molding, his processing performance is the same as the polyolefin, no need to change the process conditions, the amount of addition is generally 0.5-1.5%. It can be adjusted appropriately according to the adhesion of raw materials and the requirements of product characteristics. When using the masterbatch according to the required proportion of resin and raw materials mixed evenly into the extruder can be.
On the precipitation of the film smooth agent
Today's polymer in the flexible packaging industry has been widely used, but due to the high coefficient of friction in the film processing of polypropylene hydrocarbons in the film between each other or between the film and the processing equipment, easy to adhere to the phenomenon. Therefore, the surface of the film material is smooth and has the appropriate coefficient of friction for the film packaging process is very important, or rather, the coefficient of friction of the surface of the material is one of the main factors affecting the speed of the packaging machine and the ease of opening the package. Adding additives, such as slip agents and anti-tack agents, to the material manufacturing process is a common way to modulate the coefficient of friction of plastic surfaces.
The main purpose of a slip agent is to change the balance between film sliding and anti-adhesion by significantly reducing the coefficient of friction of the plastic film. Slip agents improve the surface properties of polyolefin films by reducing film-to-film friction (on rolls) and friction between the film and other surfaces in contact. Thus, the talc acts to 1. help increase manufacturing speed; and 2. reduce the coefficient of friction and increase the packing speed of the machine.
The talcum agent is divided into two categories according to the function of the internal talcum agent and external talcum agent: the internal talcum agent can promote the polymer macromolecule chain or chain segment relative movement, so as to improve the fluidity of the material; the external talcum agent is the polymer group with poor compatibility of the polar organic chemicals, in the polymer chain of the Brownian motion, these molecules migrate to the film surface to form a layer of oily surface, so as to play a role in improving the surface performance of film talc and reduce material friction. The molecules migrate to the surface of the film to form an oily surface under the action of Brownian motion of the polymer chains, thus playing a role in improving the surface properties of the film and reducing the coefficient of friction of the surface.
One, the reasons for the precipitation of talc
The talc is added to the talc, not grafted on the PE molecule, film processing, with the passage of time and temperature changes, the talc will be migrated from the film membrane surface outward oozing. Careful observation will be found to be a very thin layer of powder or wax-like material, with the hand to wipe can also be wiped away. The longer it takes, the more it migrates.
Commonly used talcum powder is silicone compounds, erucic acid amide, oleic acid amide and so on. The latter two are mainly used domestically, and organosilicon compounds are used less. These additives are different degrees of side effects, mainly in the organic talcum agent has a large number of precipitates on the surface of the film; and inorganic talcum agent due to the dispersion of the problem has been the production of difficult, if the formula to add lubricants and organic dispersants on the same produce precipitates. Domestic use of the slippery agent commonly precipitation serious problems, roll on the residue of a large number of white frost, affecting the surface quality of the film. Foreign countries are mainly focused on the development of silicone-based talc, such as the British Dow Corning polymerization of silicone compounds, not only has excellent talc, and less precipitation, will not produce ordinary talc due to the migration of the drawbacks brought about by the agent. In the high-grade PVC film lubricants are mainly used in Germany Henkel company produces G60, G70S and other series of products, the two series of products are mainly aliphatic polyacids and polyols polyester and saturated aliphatic alcohol polycarboxylate.
Usually, the thicker the film, the more slippery agent per unit area. Practice has shown that when the thickness of polyethylene film exceeds 60 μm, the phenomenon of additive precipitation increases considerably. This is because at the same specific surface area, as the film thickness increases, the amount of additives contained within it grows accordingly.
When the temperature is low, the PE film itself will precipitate the smooth agent, so in the production of composite rubber rollers often appear white powdery objects is the smooth agent precipitation performance.
Two, the smooth agent precipitation caused by the problem
When the smooth agent precipitation is more harmful, not only affects the work of the automatic packaging machine, but also affects the printing suitability, composite strength, and pollution of the packaged goods.
The talcum agent and polymer polyethylene is a kind of mechanical mixing, can not be very good compatibility, the molecular thermal movement so that its gradual migration to the low free energy interface, the formation of a layer of weak interface, so if the talcum agent content is too high, then it will affect the printability of the film.
Flexible packaging manufacturers in the process of film, sometimes encountered such a phenomenon: the use of ordinary polyurethane adhesive composite polyethylene film, compounded the film just off the machine, peeling strength can meet the requirements, into the drying room after aging, the strength of the contrary, greatly reduced, or put for a period of time, easy to open, broken bags. Compound film manufacturers often think that the glue problem, in fact, the cause of this phenomenon, in general, is due to the film in the slippery agent and other additives precipitation caused by.
Three, the smooth agent precipitation solution
First of all, in the production process management, just out of the production of PE film material because the smooth agent has not been completely precipitated, so should not be used just out of the production of PE film material for the composite production. PE base film should be placed for a period of time before use, such as half a month or a month, usually in about 20 days, depending on the effect of practice. This is not a special practice, there are a number of products also use this method. Such as paper wrapping film CPP, due to the paper wrapping film needs to have better anti-static properties, the general requirements of this type of film after the production of aging treatment time of about seven days, so that the role of the film added to the antistatic agent can achieve the best use of the effect. From this need to communicate with the customer, the order should be set aside enough time.
Secondly, if the PE film itself, the smooth agent precipitation is very strong, with the above method is still unable to get an effective solution, then you should notify the manufacturer to negotiate a solution, which involves a PE production formula. According to the equipment, production process, packaging requirements, the external environment, the contents and shelf-life requirements, such as adjusting the amount of slip agent and the choice of the type of slip agent, reduce the amount or change other high purity is not easy to precipitate the smooth opening agent! For example, the polymerized silicone compounds of Dow Corning mentioned earlier. In fact, the market has a good quality of smooth agent, and resin has a certain compatibility, not easy to spray frost and precipitation and other phenomena, does not affect the transparency of the film, can strictly guarantee the surface of the film micro-smoothness; In addition, compared with the traditional amide-type smooth agent, the silicone additives will not be precipitated, and does not affect the film's printing and heat sealing, is the current most ideal polymer smooth agent.
At present, the production technology level of PE film suppliers is uneven, so try to consider a certain level of production technology level of the manufacturer when selecting suppliers, and in the formal cooperation before the receipt of clearly defined standards, such as quality deviation compensation for damages, and so on.
Generally speaking, the smooth agent precipitation has nothing to do with the grade of raw material PE.