Discoloration and whitening of PVC plastic profile mold

In the production and application of profiled materials,thermo optic oxidation reaction will occur.Under the action of heat and pressure,polymer macromolecules will be oxidized by trace impurities such as water,acid,alkali and air,resulting in the reduction of molecular weight and the change of macromolecular structure.This chemical change is called degradation,and its most intuitive performance is that the surface turns yellow,the gloss gradually disappears,and even turns gray seriously.The discoloration of profiles leads to customers'doubts about the performance of PVC materials,and the consequences are serious.

1、Discoloration of profile

1.Heat,oxygen and light aging discoloration

Many people believe that to solve the problem of inconsistent color in profile processing,as long as the yellowing profile production line reduces the processing temperature,the color difference can be reduced.However,some people believe that PVC,CPE and stabilizers have the greatest impact on yellowing.Different batches of PVC,CPE,stabilizers and other main and auxiliary raw materials will have different color differences in processing.In this case,even if a certain amount of brightener and anti-aging agent are added alone,it will not play a big role.

The main manifestation of heat,oxygen and light aging discoloration is the yellowing of profiles.When promoting their products,some stabilizer manufacturers claim that using their products can reduce the number of parts and reduce the cost compared with similar manufacturers.Although the results can also produce profiles with relatively satisfactory appearance,in the recycling of waste products and in the harsh process temperature requirements,due to their insufficient heat stability efficiency,The damage to the molecular structure of PVC is not observable by the naked eye,so the following situations occur during production:

(1)It is found that the color of the profile is not uniform during processing(if there is a slight deviation in color,it is allowed);

(2)The profile obviously turns yellow after heating deformation;

(3)It is in good condition after heating,but it turns gray after the profile is made into doors and windows for a short time.Profiles do not turn yellow in normal production,but turn yellow in later window products.

In the process of profile production,when the basic conditions such as raw materials and molds have not changed,if the temperature of the plasticizing section and homogenizing section is increased by about 5℃,the resulting profile will turn yellow,which also indicates that its thermal stability efficiency is insufficient.The addition amount of heating stabilizer should be increased,especially when brightening additives such as fluorescent brightener are added to the formula,the thermal stabilizer is insufficient,and the whitening effect is not obvious.

2.Relationship between PVC molecular structure and color

(1)Because PVC resin is a kind of heat sensitive plastic,its light stability is poor.Under the action of heat and light,the branch chain takes place the dehydci reaction and polyene structure molecules.When the number of conjugated double bonds in the main chain is not too much,a small amount of color difference will occur.Hydrogen chloride will first react with the surrounding potential substances that are active to acids,and its conjugated double bonds become a new active position in the PVC molecular chain.After being initiated into macromolecular free radicals by light,PVC is prone to oxidation and color change.

(2)There are a certain number of low molecular weight components in PVC resin,which reduces the thermal stability of the polymer.The decomposition mechanism of PVC includes free radical mechanism,ionic mechanism,single molecule mechanism,etc.in addition to the stabilizer,the decomposition of PVC may also be affected by the quality of PVC resin itself,such as whether there is too much initiator left in PVC resin.

(3)If there are some impurities in the polymer,such as the initiator,catalyst,acid,alkali,etc.added in the polymerization process can not be removed completely,or water is absorbed in the storage and transportation process,the stability of the polymer will be reduced.Because these substances can cause molecular ion degradation reaction,CPE contains more low molecular substances such as Cl2 and HCl,which is easy to accelerate the thermal decomposition of the resin.Therefore,for the PVC CPE system with poor stability,the yellowing problem of some profiles can also be eliminated by increasing the amount of stabilizer.

(4)Hard PVC structural profile is a mixed system composed of multiphase polymer and mixture with PVC as continuous phase kneaded with heat stabilizer,light stabilizer lubricant,filler and pigment.When PVC profile is exposed to the natural environment,once the PVC component is dehydrochlorinated,the original component of calcium carbonate will be transformed into a new component of calcium chloride,which is dispersed in the oxidative degradation layer and forms a potential water absorption position,This is one of the reasons why outdoor profiles turn yellow.

(5)In addition to temperature stress,plastic is thermally degraded by shear stress and tensile stress in the process of high stirring,cold stirring and extrusion before molding.

When solving the color difference,a single method cannot solve the problem better.For example,it is faster to solve the problem from the following four aspects:

(1)Reducing the processing temperature makes the color whiter than not reducing the processing temperature;

(2)The effect of adding single lead salt in composite stabilizer is not better than that of adding composite stabilizer;

(3)Simply adding stabilizers is not as effective as using some stabilizers,whitening masterbatch or high whiteness filler to increase titanium dioxide;

(4)It is better to add titanium dioxide only and add some internal lubricant moderately.It is generally believed that low molecular substances have plasticizing effect on polymers.One of the mechanisms is that the interaction between macromolecules and small molecules replaces the interaction between macromolecules,so that the movement of macromolecular segments is much easier.

The above measures will prevent PVC decomposition to varying degrees and reduce the color inconsistency in profile processing.The author believes that the most effective way is to optimize the design from a stable system and fundamentally inhibit the conjugated polychromatic structure of PVC.

In production practice,the self-made UV lamp is used to age the profile for 12 hours,and there is obvious color change.It is undoubtedly necessary to carry out natural climate exposure aging test on the products in time,so as to select the formula with excellent weather resistance from the aspect of ingredients.

3.Non environmental lead sulfur pollution discoloration

It is reported that"after the PVC profile is exposed to rain and strong sunlight outdoors,the partial surface of the profile changes color.Some of the profiles on the same window have a more severe color change in the place of the remaining raindrops,and the surface color is light gray or light gray".Among other reasons,the uneven dispersion of Ultramarine as a light cyan colorant is one of the reasons.The sulfur-containing compound of Ultramarine aluminum silicate,which is alkali resistant and acid resistant,is used in excessive amounts or accumulates in a large amount in PVC profiles.Theoretically,it may produce black lead sulfide by interacting with sulfur-containing compounds,and the phenomenon of color change has appeared in many profile door and window factories.This is a noticeable problem for profile factories that use lead salt series stabilizers and add low-quality titanium dioxide and ultramarine at the same time.

Some manufacturers also reported that the profiles in contact with the rubber sealing strip also have obvious discoloration,and the discoloration extends around.In addition to the poor sealing strip containing oil materials that are easy to precipitate,the other reason is that the residual sulfur in the rubber sealing strip reacts with lead to lead sulfide.

2、Whitening of profile

1.Titanium dioxide is the best white pigment,which is widely used in plastic profiles.It is also a very good light shielding agent.Rutile(R-type)titanium dioxide has a relative density of 4.26 and a refractive index of 2.72.It can reflect or refract most of the visible light,and can completely absorb ultraviolet rays with a wavelength less than 410nm.It has good heat resistance and weather resistance,and has a strong role in shielding ultraviolet rays.It is one of the important raw materials to improve the weather resistance of profiles.At the same time,it can also inhibit the dehydrochlorination of PVC and delay the decomposition of PVC.However,due to its high market price,some profile factories in order to reduce costs,Reduce the amount of titanium dioxide to 3.6 parts,or even less,which greatly reduces the weather resistance of the profile.

It is reported in the literature that when the content of titanium dioxide in the product is 8%,its coverage tends to balance,and it is useless to add more beyond its range.Since the addition of titanium dioxide to polymer materials will increase the viscosity,reduce the fluidity,and increase the torque,resulting in processing difficulties,it is generally enough to add about 5 parts.If the whiteness is insufficient,a small amount of fluorescent brightener can be added.

Fluorescent brightener is a special powdery organic matter,which can absorb ultraviolet light waves below 400nm,convert the absorbed energy,radiate purple or blue fluorescence of 400-500nm,and use it to make up for the purple or blue wavelength lost by the reflection of the matrix.Therefore,it can be said that fluorescent brighteners not only whiten but also play a certain role in UV absorption.A small amount of blue pigment is added to white products to achieve the purpose of"covering yellow".Fluorescent brightener is to increase the number of reflected light and play the role of"brightening",which is different from the"dimming"effect of adding blue and whitening,so as to increase the surface brightness of products,make the color of products clearer and bright,and achieve the whitening effect of"radiant".

The selection of fluorescent brightener should take into account the melting point,decomposition temperature,sun fastness,solubility and maximum absorption wavelength.Domestic general brighteners such as FP type have low decomposition temperature,initial decomposition temperature of 178℃,maximum absorption wavelength of 363nm,and sublimation phenomenon.Although the price is low,the effect is not good.If type 0b is selected,the melting point is 196-203℃,the decomposition temperature is greater than 220℃,the maximum absorption wavelength is 375nm,"0B-1"type,the melting point is 353-359℃,the maximum absorption wavelength is 374nm,and the maximum emission fluorescence wavelength is 434nm;Among them,"127"is especially suitable for PVC products.