1.2.5 Physical and chemical changes during molding

A. Physical changes in polymer during molding

(a) Polymer crystallization

(1) The concept of crystallization

There are two main types of polymers: crystalline polymers and amorphous polymers. Amorphous polymers are also called amorphous polymers.

The main difference between crystalline and non-crystalline polymers: whether configuration (structural form) of molecular chain can be stably and regularly organized in process of high-temperature polymer melt into a low-temperature solid state, it can be crystalline and vice versa amorphous.

Those that can be crystallized:

①Polymers with a simple molecular structure and high symmetry, such as polyethylene.

② Although some molecular chains are relatively large, force between molecules is also very large, such as PA, POM, etc.

Difficult to crystallize:

①Polymers with large side groups in molecular chain, such as polyps, etc.

② Polymers with high molecular chain rigidity such as polysulfone, PC and PPO.

Differences in physical and mechanical properties of crystalline and non-crystalline polymers: crystalline polymers, as a rule, have heat resistance, opacity and high strength, while non-crystalline polymers are just opposite.

The difference between crystalline state of polymers and crystalline state of low molecular weight substances: mostly irregular crystals, incomplete crystallization, slow crystallization rate, and no obvious melting point.

For most crystalline polymers, total melting point (still conventionally called melting point) is Tm= (1.5-2.0)Tg

Among them, a polymer with high symmetry occupies upper limit, and vice versa, lower limit.

The parameter that makes it possible to judge ability of a polymer to crystallize is rate of crystallization.

Note. Even for polymers with high crystallization capacity, if external conditions are insufficient, crystallization rate may be low or even absent.

Indicators for evaluating crystal morphology of polymers: crystal shape, size, isotacticity and crystallinity, etc. As a rule, crystalline form of polymers is mainly spherulites, and fibrous crystals can also form under high pressure conditions.

(2) Secondary crystallization and post-crystallization

Secondary crystallization refers to phenomenon of crystallization that occurs in an imperfect part of primary crystal structure at a later stage of crystallization or in an amorphous region left after initial crystallization.

Post-crystallization is a continuous process of crystallization that occurs after part of a portion of polymer crystallizes too late.is taken after part is formed. Post-crystallization often generates and develops at primary crystal interface, which promotes further crystal growth in polymer.

Effect of annealing heat treatment on products in which secondary crystallization and post-crystallization may occur:

Both secondary crystallization and post-crystallization change properties and dimensions of product during use or storage. Annealing heat treatment is performed on formed product, so that rate of secondary crystallization and late crystallization of product can be accelerated by high annealing temperature, and crystal structure in product can be improved as soon as possible, effectively ensuring productivity and size of product when it leaves factory.

(3) Effect of crystallization on polymer properties

①Density

Crystallization means that molecular chains are arranged in a regular and compact configuration, intermolecular forces are strong, and density increases with increasing crystallinity.

②Tensile Strength

Due to strengthening of force between polymer macromolecules after crystallization, tensile strength also increases with increasing crystallinity.

③Impact strength

The impact strength of crystalline polymers is lower than that of amorphous polymers due to regular arrangement of molecular chains.

④Modulus of elasticity

The elastic modulus of a crystalline polymer is less than that of an amorphous polymer.

⑤Thermal characteristics

Crystallization helps to increase softening point and thermal bending temperature of polymer.

⑥Fragility

Crystallization will shorten cooling time of polymer in injection mold, so that molded product will have a certain degree of brittleness.

⑦Deformation

After crystallization, volume of polymer decreases due to correct arrangement of molecular chains. The higher degree of crystallinity, greater volumetric shrinkage. Crystalline parts are more prone to buckling due to uneven shrinkage than amorphous parts, which is caused by uneven crystallization of polymer in mold.

⑧Roughness and transparency of surface

The correct arrangement of molecular chains after crystallization will increase compactness of polymer structure, and surface roughness of workpiece will decrease, but transparency will decrease or be lost, since spherulites will cause light waves to scatter. The transparency of polymers is due to amorphous arrangement of molecular chains.

(4) Crystallization rate and crystallinity

①Crystallization rate

Reflects crystallization ability of crystalline polymers in external conditions.

The melt temperature ranges from Tm to Tg, and crystallization rate is mainly affected by temperature.

The crystallization process consists of two stages: nucleation and growth. Homogeneous embryo ratepolymer formation increases with decreasing temperature, but crystal growth rate decreases with decreasing temperature, so crystallization rate of various crystalline polymers can reach a maximum at a certain temperature, write As θcmax, use following empirical formula to estimate .

θcmax=(0.80–0.85)Tm

Even if temperature is always maintained at θcmax, it still takes a long time for polymer to fully crystallize. In manufacturing, semi-crystallization period (the time required to reach 50% crystallinity) or crystallization rate constant is often used as an indicator to evaluate crystallization rate and crystallization ability.

②Crystallinity

Crystallinity refers to ratio of mass (or volume) of crystal structure in a polymer to total mass (or volume) of polymer. Characterizes degree of crystallinity of polymer.

The maximum crystallinity is related to its own structure and external conditions (such as temperature, etc.). The crystallinity of most polymers is about 10-60%, but crystallinity of some polymers can reach very high values, such as polypropylene, crystallinity reaches 70-95%, and crystallinity of HDPE and PTFE can also exceed 90%.

(5) Factors affecting crystallization

The crystallization of polymers under isothermal conditions is called static crystallization, and process of crystallization under non-isothermal conditions is called dynamic crystallization. In injection molding, dynamic crystallization of a polymer is affected by its own organization and structure, heating and cooling, stress, time, and other process conditions.

①Melting temperature and time

Crystalline polymers may have more or less crystalline structures in their state of aggregation prior to heating and shaping. When polymer is heated above Tm, these structures may become heterogeneous nucleators during cooling and crystallization of crystalline preform. However, as heating temperature increases and heating time increases, these original crystal structures can be destroyed by molecular thermal motion, so heterogeneous nucleation crystal blanks will decrease or disappear.

Crystallization in process of polymer molding manifests itself in two main ways: first is that under conditions of high melting temperature and long melting time, all crystal structures remaining in melt are completely destroyed, and only upon cooling and crystallization Crystal nucleation is carried out by supercooling or supersaturation (this phenomenon is called spontaneous nucleation or homogeneous nucleation), and then gradually increases; other occurs under conditions of lower melting temperature and shorterwhom melting time. and then gradually grow up. The homogeneous nucleation crystallization rate is low and crystal size is large, while heterogeneous nucleation crystallization rate is high, crystal size is small and uniform. When molded with a lower injection temperature and a shorter injection time, it is beneficial to promote heterogeneous nucleation and crystallization, and also helps to improve mechanical strength, heat distortion temperature, and wear resistance of part.

②Cooling rate

Among various external factors affecting polymer crystallization, cooling rate has greatest influence, due to which crystallization rate can differ by several times and even tens of times. The cooling rate depends on temperature difference Δθ (also called subcooling) between melt and mold, when melt temperature is constant, Δθ is determined by mold temperature θm.

Ⅰ When θm is close to θcmax, degree of supercooling is small and cooling rate is low (so-called slow cooling). Crystallization characteristics are similar to isothermal static crystallization, i.e., starting from homogeneous nucleation, larger spherulitic structures are formed in workpiece. Although a large degree of crystallinity can be obtained, crystal structure makes workpiece brittle and reduces its strength, and workpiece is easily warped and deformed due to insufficient cooling. In addition, slow cooling also impedes productivity, so slow cooling molding is rarely used in production.

Ⅱ When θm is much lower than Tg, degree of supercooling is very large and cooling rate is high (so-called rapid cooling). The macromolecular chains line up regularly too late, forming a loose disordered structure, or forming some microcrystalline structures only in center of thicker part, resulting in uneven crystallization of part. Internal stress should not be used, especially for parts with high crystallization ability and low glass transition temperature, such as like PP, PE and POM.

Ⅲ. If Tm is controlled in a temperature range not too high above Tg, degree of supercooling will not be too large, and crystallization rate and billet cooling rate will be moderate (so-called medium cooling). The polymer melt first cools on surface of mold cavity and forms a thin shell, and then crystallization begins with a thin shell and gradually goes deep into part. The cooling thin shell plays a certain role of thermal insulation, and melt inside part is kept in crystallization temperature range for a long time, which is conducive to initiation of crystallization and growth of crystals inside part, and can also improve crystallinity inside part and promote crystal structure,usually complete.

The molded parts are cooled at a medium speed, crystallization structure is relatively stable, crystallization stress is small, it does not cause size and shape changes, and production cycle is shorter than that of slow cooling. The production usually uses medium speed chill molding and mold temperature is controlled between Tg and θcmax.

That is: as cooling rate increases, crystallization time of polymer decreases, crystallization temperature decreases, crystallinity decreases, and density of part decreases. On example of polyethylene, effect of cooling rate on density is shown in fig. 1.17.

Figure 1.17 Effect of cooling rate on density

③Shear stress and pressure

Influence of shear stress on crystallization rate and crystallinity:

During molding, flow and deformation of polymer melt depend on shear stress and shear rate. The macromolecules straighten out in direction of stress and form a flow orientation structure. The orientation structure is important for heterogeneous nucleation and crystallization. during crystallization Growth can be induced and stimulated. Increasing shear stress or shear rate increases degree of orientation of polymer and increases rate of crystallization and crystallinity of polymer. However, if shear stress is applied for a long time, deformation and relaxation will reduce or disappear orientational structure, and crystallization rate will decrease again.

Influence of pressure on crystallinity:

As pressure increases, crystallization temperature of polymer will increase, respectively, crystallinity will increase and, accordingly, density will increase (Fig. 1.18).

Figure 1.18 Effect of crystallization rate on density

The effect of shear stress on crystal morphology and polymer structure:

The rotating plasticizing effect of screw causes polymer melt to undergo relatively strong shear, and it is difficult to form larger spherulites in melt, and crystal structure will be quickly crushed into small crystal nuclei. crystal structure is relatively uniform and fine; this effect cannot be achieved if a plunger injection molding machine is used for injection molding.

Influence of molding pressure on crystal morphology:

Large and full spherulites are easily formed at low pressure, while small and irregular spherulites are easily formed at high pressure.

④Molecular structure, low molecular weight substances and solid impurities

The chain structure and relative molecular weight of polymer macromolecules are main factors that determine crystalline and non-crystalline polymers.

Ⅰ. Influence of chain structure of polymer macromolecules on ability to crystallize and process of crystallization:

The structure of macromolecular chain is simple, molecular chain is small, degree of branching is low (no or only a few branches in main chain), chemical structure of molecule is symmetrical, stereoregularity is good, rigidity and flexibility of macromolecule and intermolecular strength are moderate, both of them are useful for increasing speed crystallization and crystallinity.

Ⅱ. Influence of relative molecular weight of polymer on movement of rearrangement of segment structure:

When relative molecular weight is large, rearrangement movement is difficult and crystallization ability is reduced, otherwise crystallization ability is increased.

Ⅲ. Effect of low molecular weight substances (such as solvents and plasticizers, etc.), water molecules and solid impurities added to polymers, on crystallization:

After carbon tetrachloride solvent diffuses into polymer, it accelerates crystallization process in area with internal stress, polyamide with high hygroscopicity, etc. accelerates crystallization of surface of workpiece after absorbing water; substances (nucleating agents) in crystal core, such as carbon black, talc, silicon dioxide, titanium oxide, and polymer powder, can greatly increase crystallization rate.

(b) Orientation of polymer molecules

(1) Orientation concept

Polymer orientation is an ordered arrangement of polymer macromolecules and segments of its chain or microcrystalline particles of a crystalline polymer under stress, which is called orientational structure.

Figure 1.19 Schematic diagram of orientation of molding process

Orientation classification:

①According to stress properties, orientation structure is divided into tension orientation and flow orientation. The first is due to torsion and its orientation corresponds to direction of stress, second is formed along direction of melt flow under action of shear stress. In extrusion, there is predominantly a stretching orientation, and in injection molding, a predominantly flow orientation.

②According to melt flow properties, orientation structure is divided into single-axis orientation and multi-axis orientation (or flat orientation), see Figure 1.19. In a uniaxial orientation, structural units are ordered along one direction of flow; in a multiaxial orientation, structural units can be ordered along two or more directions of flow.

③According to temperature distribution and changes in melt flow, orientation structure is divided into isothermal orientation and non-isothermal orientation. The orientation in barrel and nozzle can be roughly considered as an isothermal orientation, and orientation in various channels, gates, and mold cavities as a non-isothermal orientation. The non-isothermal orientation of runner, gate, and cavity has a major impact on surface quality and part performance.

④According to difference between crystalline and non-crystalline polymers, orientation is divided into crystalline orientation and non-crystalline orientation.

(2) Orientation mechanism

The orientation of amorphous polymer macromolecules (including flow orientation and stretch orientation) is of two types: segment orientation and molecular chain orientation.

Segment orientation can be achieved by segment motion caused by internal rotation of single bonds, which can be done in a highly elastic state.

Orientation of macromolecular chains can be realized only due to coordinated movement of each segment of chain of macromolecules, which can only be carried out in a viscous-flow state.

The orientation process is process of movement of chain segment, which must overcome resistance of viscosity inside polymer. The resistance of two types of units of movement, segment and macromolecular chain, is different, so speed of orientation process is also different. Under action of an external force, first orientation of chain segments occurs, and then it develops into orientation of macromolecular chains.

The process of orientation is process of ordering macromolecular chains or segments, which can only be realized under action of an external force field. The process of deorientation is that macromolecules tend towards disorder and disorder, which is a spontaneous process. The orientation state is a non-equilibrium state in thermodynamics, when an external force is removed, segments orand molecular chains spontaneously deorient and return to their original state.

In order to obtain oriented materials, it is necessary to quickly lower temperature below glass transition temperature after orientation in order to freeze movement of molecular chains or segments. Of course, such freezing refers to a thermodynamically non-equilibrium state, only relative stability, with increasing time, especially with increasing temperature or swelling of polymer with a solvent, deorientation still occurs.

(c) Flow Orientation Mechanism During Injection Molding

(1) Polymer flow orientation

At initial stage of mold filling, melt flows from sprue into mold cavity, and material flow is radial, forming a structure with a flat orientation. A schematic orientation diagram is shown in Figure 1.20. After melt contacts surface wall of mold cavity, process of filling mold begins. The melt that first contacts surface wall of cavity cools rapidly, forming a thin shell that is too late to orient, and subsequent melt will flow. in a thin shell.

Figure 1.20 Orientation of polymer flow in tube and mold

Due to friction of a thin shell against melt, resistance to flow of melt near it is very high, and melt creates a large shear stress, so macromolecules are strongly oriented here. At same time, middle part of melt experiences least friction, shear stresses are not too high, and orientation is weak. In transition between melt in middle and melt near thin shell, degree of orientation of macromolecules is moderate. Between first two degrees of orientation, orientation in melt front is due to stagnation. form wall.

Size distribution of degree of orientation of polymer in direction of flow:

The shear stress at orientation at any point in flow direction is proportional to pressure of melt flow. Since flow pressure gradually decreases from gate to melt front, degree of orientation on gate is greatest, and then gradually decreases.

In addition, along flow direction, temperature of melt continuously decreases, so thickness of thin shell in contact with surface of cavity wall, but not subjected to orientation, will gradually increase.

(2) Solid filler flow orientation

The behavior of solid filler in mold cavity is very complex, but orientation structure always follows flow direction.

Figure 1.21 is a schematic diagram of flow orientation of a fan-shaped fiber-filled sheet product, and melt streamline extends radially from gate. The flow velocity is maximum at center of fan-shaped cavity, when melt front reaches surface of cavity wall and is forced to change flow direction, melt turns in both directions, forming a flow perpendicular to radial one. If melt contains fibrous fillers, they will also change flow direction along streamline and, finally, will form an arcuate structure, location being most obvious at edge of fan-shaped portions. a relatively complex planar orientational structure appears.

Figure 1.21 Schematic diagram of flow orientation of fibrous fillers in fan-shaped products

(3) Influence of Orientation on Polymer Properties

Table 1.6 Anisotropy of some polymers after stretch orientation

For non-crystalline polymers, orientation is ordered arrangement of macromolecules and their chain segments. After orientation, polymer exhibits a clear anisotropy. The mechanical properties in orientation direction are significantly improved, and mechanical properties perpendicular to direction orientation are significantly reduced. Table 1.6 shows anisotropy of some polymers after stretch orientation. Although stress properties of stretch orientation and flow orientation are different, their effect on mechanical properties can still be used as a guide.

The mechanical properties of crystalline polymers are also anisotropic due to orientation, but differ slightly from those of amorphous polymers:

①The orientation of crystalline polymers is result of stretching of molecular bundles of microfilaments connected to plate. In orientation of orientation, mechanical strength and density can be improved, and elasticity and toughness will also be improved, but elongation is lower due to macromolecules Regular arrangement has decreased.

②The orientation effect of crystalline polymers can only be effective at temperatures below melting point and above crystallization temperature. When crystallization temperature is lower than crystallization temperature, polymer cannot withstand shear flow, so there is no orientation effect.

The elastic modulus increases, Tg increases (for highly oriented or highly crystalline polymers, Tg can increase by about 25°C), coefficient of linear expansion increases along orientation, and shrinkage rate is proportional to degree of orientation.

The degree of polymer anisotropy after orientation is related to flow properties during orientation:

The degree of anisotropy in uniaxial orientation is most obvious, while biaxial (or multiaxial) orientation can reduce anisotropy. In addition, degree of anisotropy after uniaxial orientation of amorphous polymers is more significant than that of crystalline polymers.

(4) Factors Affecting Orientation in Injection Molding

①Temperature

The degree of orientation decreases with increasing melt temperature Tf and mold temperature θM. This is due to fact that an increase in temperature has a beneficial effect on deformation and flow of melt, and degree of orientation may increase, but deorientation ability of polymer increases faster. decrease in degree of orientation.

Deorientation: refers to ability of oriented polymer macromolecules to return to their original twisted state due to Brownian motion at high temperature. The process by which macromolecules return to their original shape is called macromolecular relaxation.

Influence of temperature on orientation:

Ⅰ.If melt temperature is very high, it means that there is a wide range between it and solidification temperature,and therefore relaxation time of macromolecules increases, therefore disorientation ability increases, and degree of directionality decreases.

II. The relaxation time of amorphous polymer macromolecules refers to process when melt temperature Tr drops to Tg, and relaxation time of crystalline polymers refers to process when Tr drops to θM. The relaxation time of former is longer than that of latter (θM>Tg), therefore, in process of relaxation, cooling rate of crystalline polymer is high, and movement of macromolecules is easy to freeze, so a higher degree of orientation can be obtained.

②Injection pressure and holding pressure

Increasing injection pressure and holding pressure can increase shear stress and shear rate in melt, help speed up orientation process, and increase degree of product orientation and density.

③Gate Freeze Time

After melt fills mold cavity and stops flowing, molecular thermal motion will still be relatively intense for a long time, and spread and oriented structural units are likely to be disoriented. However, with a large gate, melt in gate cools slowly, gate solidifies later, and flow process will be slow, which can compensate to a certain extent for misorientation caused by thermal movement of macromolecules, especially during pouring process. Near mouth, orientation is very pronounced.

④Form temperature

When mold temperature is low, movement of polymer macromolecules is easily frozen, deorientation ability is reduced, and degree of orientation is increased.

⑤Fill Rate

Ⅰ. Fast form filling. Due to effect of flow velocity, surface of part is highly oriented and core temperature drops much more slowly than with normal mold filling. The ability to deorient is increased, and degree of orientation is somewhat lower than at surface.

Ⅱ, slow mold filling, contact time between melt and surrounding interface (gutter and wall of cavity surface, etc.) is long, mold dissipates more heat compared to fast filling at same injection temperature, relaxation time of macromolecules is shortened , ability to disorientate decreases and degree of orientation improves. In addition, when filling mold at low speed, more injection pressure is often required, so degree of orientation will be further improved.

In addition, some of physical properties of polymer also affect orientation. For example, at high specific heat and latent heat of polymer crystallization, cooling rate slows down, disorientation ability increases, and degree of orientation decreases.

(c) Influence of residual stress on injection molded products

Residual stress occurs during flow and cooling of melt in mold. Residual stress near neutral layer thickness of plastic parttensile stress, and surface layer, compressive stress.

(1) During injection and pressure holding step, plastic is subjected to unbalanced high shear and normal stress, which causes a residual stress latent in plastic part, called flow residual stress.

(2) Due to uneven temperature of injection mold and rapid cooling and solidification of plastic parts in mold, thermal stress of plastic parts is generated by temperature difference, which is called residual temperature. stress.

The thicker plastic part, greater residual thermal stress and lower residual flow stress. The distribution of residual stresses is also related to process and conditions of injection process. The area near gate has highest residual stress.

Residual stress will cause plastic parts to change shape and size. During cooling process of plastic part in mold, although orientation and residual stress already exists, it is constrained by mold wall, not constrained during cooling process after demolding, but subject to various disturbances. The tensile stress and compressive stress on wall thickness of plastic part, as well as internal stress of each part of plastic part, will lose balance with each other.

Effects caused by residual stress:

When stress in a certain area of ​​the plastic part exceeds limiting stress of material, various deformations occur and molded body of plastic part is destroyed; when residual stress of plastic part exceeds limit stress of material, surface of part will have various cracks.

Also:

(1) Micro-shrinkage of crystallized plastic parts at a later stage, interference shaft and hole assembly, and stress and residual stress caused by compressive force of threaded fastening are superimposed on each other.

(2) Residual creep and relaxation stresses of plastic parts change under long-term loading.

(3) Under such long-term environmental conditions, temperature changes lead to fluctuations in thermal stress. This will cause changes in internal stresses and even lead to an imbalance in overall stress balance, so that warpage deformation will develop and increase.

B. Chemical changes in polymer during molding

Chemical reactions mainly include splitting and crosslinking.

The crosslinking reaction is an important molding process for thermosetting polymers. Without crosslinking reaction, thermosetting polymer cannot change linear structure to body structure, and parts cannot be cured and formed; but when molding thermoplastic polymers, deviations from norm should be avoided. Crosslinking reaction impairs formability after crosslinkingvaniya.

Decomposition is usually detrimental, degrading many properties of part, making it difficult to control molding process, but if necessary, decomposition can be deliberately used to reduce viscosity of polymer melt in order to improve flow and molding properties.

(a) Degradation

(1) Degradation mechanism

The chemical degradation reaction of polymers under influence of external conditions such as high temperature, stress, oxygen and moisture can lead to a number of structural changes, such as breaking polymer molecular chain and reducing relative molecular weight, and thus make polymer disappear elasticity, decrease in strength, change in viscosity, turbulent flow of melt, surface roughness of product, reduction in service life.

Properties of chemical reaction of decomposition process can be divided into:

Free radical chain degradation and random degradation.

①Characteristics of free radical chain degradation (depolymerization reaction):

When heat and force during molding reach or exceed chemical bond energy of polymer, some chemical bonds in backbone of macromolecule will break and generate initial free radicals, and then through formation of active sites, chain transfer and chain shortening Reactions such as chain termination and chain termination, complete entire degradation process, and finally form various degradation products such as linear degradation products, branched chain degradation products, and cross-linked degradation products.

②Random degradation characteristics:

The main chain breaks randomly, and decomposition reaction proceeds in stages. Each reaction step is independent. The intermediate product formed during reaction is stable, and relative molecular weight of polymer decreases after decomposition. completed. Accidental degradation often occurs at high temperature and when polymer contains traces of moisture or impurities such as acids and bases.

If there are certain impurities in polymer (for example, initiators, catalysts added during polymerization of agents, acids and alkalis, etc.), or absorb moisture during storage and transportation, and when mixed with various chemical and mechanical impurities, all of them will catalyze decomposition.

(2), degradation types

①Thermal degradation

Thermal degradation is a degradation reaction caused by heating a polymer at a high temperature for a long time in an injection molding process, which is called thermal degradation.

The property of thermal decomposition refers to reaction of free radical chain depolymerization, and reaction rate increases with increasing temperature. Generally speaking, phenomenon of thermal decomposition of polymers caused by excessive heating temperature also belongs to field of thermal decomposition.

Thermal heat temperaturedecomposition: slightly higher than thermal decomposition temperature, because at beginning of thermal decomposition, only some unstable molecular chains in polymer are destroyed, and molecular chains are not immediately broken.

Thermal stability temperature: In terms of reliability of injection molding production, thermal decomposition temperature is often used as thermal stability temperature in production, and upper limit of temperature when polymer is heated cannot exceed thermal decomposition temperature. temperature (see table. 1.7).

Table 1.7 Thermal decomposition temperature of commonly used polymers

②Oxidative degradation

Oxidative degradation is that after contact of polymer with atmospheric oxygen, some parts with weaker chemical chains often form extremely unstable peroxide structures that easily decompose with formation of free radicals, which leads to polymer depolymerization. oxidation is called oxidative degradation.

Thermo-oxidative degradation occurs without heating and ultraviolet radiation, and reaction process of oxidative degradation of polymers is extremely slow. Injection molding is carried out at a high temperature. If temperature is not properly controlled, oxidative degradation during molding process will be rapidly enhanced by heat. In manufacturing, such a rapid oxidative degradation at high temperature is often referred to as thermal oxidative degradation.

Thermal degradation of polymers is basically similar to thermal-oxidative degradation, but meaning of thermal degradation is wider.

The rate of thermal-oxidative degradation is related to following factors:

Ⅰ Polymer structure. For example, process of thermo-oxidative degradation of polymers with saturated carbon chains is slow, and formation of peroxides is difficult, but with weaker chemical bonds in main chain, probability of formation of peroxides increases, and thermo-oxidative decomposition is accelerated, unsaturated carbon chains The double chains of polymer backbone are easily oxidized, and rate its thermal-oxidative degradation is much higher than that of polymers with saturated carbon chains.

Ⅱ, oxygen content, heating temperature and heating time. When oxygen content increases, temperature rises, and heating time increases, thermo-oxidative degradation rate can be rapidly increased. During injection molding, injection temperature and injection time must be strictly controlled to avoid oxidative degradation of polymer. due to overheating.

③、Water decomposition

Decomposition by water means that if molecular structure of polymer contains carbon heterochain groups or oxidized groups that are easily hydrolyzed, these groups are easily decomposed by water in polymer at injection temperature and pressure during production. This phenomenon is called water degradation.

Impact of water degradation:

If above different groups are located on main chain of polymer, then average relative molecular weight of polymer after decomposition by water will decrease, and mechanical properties of product will deteriorate; if these groups are located on a branched chain water will decompose. It changes only part of chemical composition of polymer and has little effect on relative molecular weight and characteristics of product.

Measures to prevent water degradation:

Take necessary measures to dry molding motherbefore injection molding or in a hopper, which is especially important for raw materials such as polyester, polyester and polyamide with high hygroscopicity.

④、Stress degradation

Stress degradation is that during molding process, molecular chain of polymer is also broken under certain stress conditions, which leads to a decrease in relative molecular weight. This phenomenon is commonly referred to as degradation under stress.

Stress degradation also refers to nature of free radical chain degradation. When stress degradation occurs, it is often accompanied by heat release, and if heat cannot be dissipated in a timely manner, then thermal degradation can occur simultaneously.

Characteristics of stress degradation:

Ⅰ When relative molecular weight of polymer is large or macromolecule contains unsaturated double bonds, stress degradation may occur.

Ⅱ. The higher voltage value, higher degradation rate and shorter final broken molecular chain.

Ⅲ. When voltage value is constant, chain break length is also constant. When length of all breaks in molecular chain reaches length allowed by voltage value, degradation process stops.

Ⅳ Increasing temperature or adding a plasticizer can reduce tendency to stress degradation. In injection molding, except in a few special cases, stress degradation is generally undesirable.

(3) Measures to prevent degradation

①Strictly control technical performance of raw materials to avoid catalysis of degradation due to raw material impurities.

②Before production, molding materials must be preheated and dried, and water content must be strictly controlled so as not to exceed value required by process and product characteristics.

Figure 1.22 Rigid PVC molding temperature range

④The molding equipment and molds should be in good structural condition, there should be no dead corners and gaps on parts in contact with resin, length of flow path should be moderate, and heating and cooling system should have a display device with high sensitivity to ensure good control temperature and cooling efficiency.

⑤ For polymers with poor thermal and oxygen stability, consider adding stabilizers and antioxidants to formula to increase polymer's resistance to degradation.

(b) Cross references

Crosslinking is process of a chemical reaction during which linear structure of a polymer is converted into a three-dimensional structure, which is called crosslinking. The crosslinking reaction is result of a reaction between reactive groups (such as methylol, etc.) or reactive sites (unsaturated bonds) in molecular chain of polymer and crosslinker.

Benefits of cross-references:

After crosslinking, strength, heat resistance, chemical stability and dimensional stability of polymer can be improved compared to original. The crosslinking reaction is mainly used in molding and curing processes of thermosetting polymers; for thermoplastic polymers, cross-linking should be avoided whenever possible, as it is unfavorable for flow and molding and affects product characteristics.

Some explanation about hardening and maturation:

Cutting means curing or curing. "Well cured" or "fully cured" does not mean that crosslinking reaction is complete, but means that crosslinking reaction during molding and curing process has reached optimum level, and parts can get best physical properties. and mechanical properties. Often, for various reasons, polymers are difficult to fully crosslink, but degree of curing can be over 100%. In production, degree of hardening exceeding 100% is called overripe, otherwise - immature. Note that for different thermosets, even if same type or grade of resin is used, when different additives are added, degree of crosslinking reaction at full cure is also different.

Relationship between solidification degree, solidification time and part characteristics:

A. When curing time is short, parts are likely to be undercooked (underhardened), and there will be more soluble small molecular substances inside, and intermolecular bond is not strong, resulting in strength and heat resistance. parts., chemical resistance and insulation index decrease, thermal expansion, post-shrinkage, residual stresses, creep and other indicators increase, surface of part loses its luster, shape is deformed, and even cracks appear. (The cause of cracks in part can be consideredon one hand, from conditions of process or form, and it can also be caused by an incorrect ratio of polymer and various additives.)

B. If hardening time is increased, piece will be overcooked (too hardened). The indicators of overcooked parts are also not best, such as low strength, brittleness, discoloration, dense bubbles on surface, etc., and sometimes even charring or degradation. (The reason for overcooking workpiece is improper molding conditions. The main reason may be too high molding temperature, temperature difference inside mold, too large and thick workpiece, etc.).

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